Hi,
underdash handbrakes, aren`t too tricky, but you will need a certain amount of access to the centre tunnel ie. cutting lumps out of it. My first attempt involved a camper underdash handbrake, and a golf handbrake cable. It worked, but wasn`t brilliant.
Attempt two, works brilliantly. You need to use a 356 handbrake and the actuating arm. Use standard length handbrake cables and cross piece. Drill out the cente of the cross piece and fit a long length of 8mm studding.
Fit the handbrake under the dash, drill the hole for the cable from the handbrake through the bulkhead.
Under the wheel arch drill a hole probably about 2" through the centre tunnel.
The actuating arm needs to be modified to use. Cut the upright in half, turn it around and weld back on. This gives you the movement you need. The tube section, will hit on the master cylinder. Cut it where it does, move it out by 2" and weld piece of angle iron underneath. Voila, clearance.
Fabricate a bracket for inside the tunnel for the other end of the round tube. Bolt In. Bolt a piece of angle iron on for the outside bracket, mount to the torsion tube at the front, with an exhaust clamp.
Fit the 8mm studding to the hole in the round tube inside the tunnel, and adjust.
You will also need to reweld the handrake conduit lower in the tunnel as the cable is now basically at floor level.
Make up a plate with a hole in it, and a rubber seal for where the round tube enters the tunnel from the under the wing.
You will obviously need to cut holes in the tunnel, one where the cross piece is, for adjustment, one at at the front for fitting the actuator.
These pics might help.
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It`s easier to do, than write about!