Hi mate, ive just started doing a similar thing

Im making a track car out of a mk3 XR3i, but i just bought a non running S2 (Non running because of a gearbox prob).

I plan to fit the engine, dash and loom out of the S2, but the gearbox casings and mounts are different, so an S2 box wont fit.

The S2 interior should fit in a Mk3, maybe with a few minor changes (eg the door cards on an s2 clip and screw on slightly differently to the MK3 XR ones)

Also the rear seat in my XR has a single peice back, so folds down in one. The S2 has a two peice one so needs the extra bracket near the middle. I dont know how difficult it would be to fit this bracket?

Dont know about the loom behind the clocks, i am pretty certain the multiplugs will differ but it cant be too much trouble to put S2 plugs on, with the help of wiring diagrams...

I havent really been much help have i?