Hi Andrew, hope you're well
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Best thing to do as a starting point is simply have a good grope around on the front inside of the wheel arch area. If the kidney bowl is going to go, it'll start at the front, right by the 'shelf' formed the rear of the outer sill. Mud piles up there, and it just asks for trouble. An external sign of problems is at the base of the B-post, as it curves into the sill. The B-post effectively ends up double-skinning the top of the outer sill for an inch or so, and generally if that's beginning to blow, the kidney bowl will be on the way out.
I'll write the following a) as a bit of an overview, but b) to underline what Nick has rightly said, it's a reasonable undertaking and once you're underway, you can't go back. It's not really a first DIY welding project, but even then you can do wonders by taking things slow and steady with lots of double checks and safety checks. Plenty of DDK'ers have done fantastic 'DIY' panel work, and I'm afraid there are a few 'specialists' that I wouldn't have weld a wheelbarrow
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Your repair method depends on the rear wings / quarters. If they are good, and I guess they are, then you'll be opening up a minimum area to repair the outer sills and kidney bowls. The trick here is to remove a section of rear wing in a way that encourages a neat re-assembly. I have the luxury of simply making a custom section every time I do this repair, and can back-mark from the door to get the shut lines I want. It's sometimes possible to re-use the wing section, but be aware that the front section of it (near the jacking point) is often pretty nasty. If the wings are toast, at least you're getting excellent kidney bowl access, but whole world of issues elsewhere.
For outer sill work you'll need to remove the front wings, but before you do anything at all, you need to make a note of your door to rear wing, door to front wing and door to sill gaps. If you have trouble with your front window frames rubbing the tops of your A-pillars, the rear of the car is low, and now is the time to address that, probably on a jig, or at least a quick pull bench. Hopefully all is well, and your gaps look good.
You need to get the car jacked up and supported to exactly reproduce the gaps it has on the ground.
1) Jack under engine, and rest car on rear torsion tube ends with normal axle stands.
2) Jack front of car, and with screw (not pin) type stands, or pin stands with fine shims, support the front of car on front a-arm area so that as you let central trolley jack down, no one stand is taking more weight. Ie you have not induced twist.
3) Using scissor jack and supporting blocks, jack near rear of engine until door gaps return to where they started, and they operate as normal, with no binding or dropping.
You now have a safely supported car. If you've ever seen a car being welded just on pin-type stands with no shims and a hanging rear end, it's now quite likely to have taken the twist of the garage floor, and some droop from the weight of the over-hanging engine
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Remove sill trims, door seal, carpet trim, carpet near sill join at top, and be ready to remove door to do a nice job. If so, mark location at A-post, and if removing by un-bolting, remove top bolt from each hinge and replace with an 8mm 'screw' made from 8mm bolt with head removed and a slot cut in it. This gives you something to initially hang the door onto when re-fitting. Alternative is to use a hinge pin remover if you can find (or make) one. You really only need to remove the door to complete and tidy the welds right at the front, so don't be I too much of a rush, and have help to hand if they are built up. They are heavy and awkward.
If you replace outer sill, you'll need to put in a new B-post base. I go in just below the pressed 'fold' below the latch area. Don't be tempted to replace the whole thing unless you have to. Having a big chunk of B-post in place is a valuable asset in returning the car to its pre-repair state.
Removing outer sill comes down to removing B-post section and removing section of rear wing. After that you'd drill out the string of spot welds along the top of the sill (literally on that top section usually hidden by the carpet trim), plus in the same area, the welds at the rear of the door seal channel. There are three or four spots at the front of the sill by the A-post (down from the check strap), one on the face of the step next to these welds, a couple more on the step itself just forward again, and then a string down the base of the wing closer. Then all the way along the bottom, and up and around the kidney bowl. Be aware that you'll want a fairly long reach spot drill to do the ones inside the rubber seal channel.
Any stubborn bits can be dealt with with a sharp, small cold chisel: you're not brutalising anything here, just teasing out the edges of any slightly mis-drilled spot welds. Sometimes it's easier to cut with a grinder (I won't go into the elves of safety here) the bulk of the sill out of the way, and then go back in to remove the flanges. If you do this, be very careful when cutting the base: it's very, very close to the inner sill here, and it's easy to catch it.
Anyway, that should get you into the sill area. It'll then be clear what the kidney bowl and inner sills are like.
Important! Only do one side at a time. That way a) you'll always have a reference, and b) you've kept as much strength in the car as possible. It's very easy to get causal about this, and the. End up with a car that doesn't quite look 'right' at the end of it.
I'd say that this could be a skilled DIY job, UNLESS:
1) If the car is known to be bent, has iffy gaps (especially ones that are much bigger at the top than the bottom rear of doors) or has the front window frames rubbing on the top of the A-pillars, you need it jigging.
2) If you can't do a reliable plug weld. On a test piece, a sample weld should be shiny on top, nearly level and have a pimple on the substrate layer underneath, and withstand destruction tests. If your welder is between 120-160 amps, it needs to be on maximum, or one down from maximum on 18 (sill) gauge metal.
3) If you can t take your time and allow twenty hours per side for the whole process of outer sill, kidney bowl and rear wing re-attachment. I can do them quicker (obviously), but that's certainly not a case of being better, and a DIY'er taking their time is something to be praised. After six months, whether something took two weekends or six will be forgotten, but the quality of the fit and finish will hang around for decades.
Pit-falls:
a) I'd use genuine sills only: the pattern products are garbage, and just make a DIY job much harder, event the inner sills need reworking to allow the outer sill to fit in the right place: avoid.
b) The sills currently available are 993 ones: they have extra pressings and holes. If you're running sill trims, these hide them. If not, they'll require re-working.
c) You can get genuine and pattern B-posts. These can both be used, and most of it goes in the bin, so might decide to go pattern. If you do, be aware that they'll require a bit of reworking on the offside, and a ton of reworking to the near side (they are too narrow).
d) Be prepared to end up doing inner sills, and therefore possibly jacking points and reinforcements. This is why we supported the car very well right from the start. The actual welding of the inner sills is easier than the outers, but joining in near the jacking point strongly requires some thinking about.
We can deal with the intricacies of the actual welding another time: there's a ton of stuff to think about when it comes to getting the outer sill into the right place (tight and neat), and getting all gaps back to where they should be.
By far the hardest thing though, is getting that rear wing back together without major distortion and wonky gaps. I have tons of pictures of sills as you'd expect: will investigate a new picture host, and get some up
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