Hi mark,
First thing you will need is a three point belt in the back, no lap belt is adequate to restrain a child seat. You can use belts from a later 911 on the parcel shelf. Make sure your shelf is not rusty and that substantial reinforcement is added to the underside of the shelf where the fixings go through.
I have resigned myself to not being able to take more than one of my kids in the car until I have my parcel shelf replaced next time I restore the car
Britax seats are generally excellent and undergo real testing and development way in excess of the required standard. I believe the Skoda test is an example of what happens when the seat does not fit. The key thing you need to consider is the shape of the child seat base. If it doesn't fit to the car securely and in a stable manner before you even get the belt on it won't stand a chance of staying put in an accident. If this seat you mentioned is an ISOFIX seat it is highly unlikely to fit as the mountings at the rear of the seat will foul on the seat back even when retracted.
Unfortunately there is no easy answer to this one, the seat shape in the back of a 911 is very unusual and choice is limited.
quickfitsbs have a good reputation and know early 911s, so this maybe the only option!