The whole rear beam is different as is the diff, the Cosworth being a 7.5" LSD (Assuming it's a 2WD Cosworth) where as the 3DR will be a 7" open diff.

The ARB'S will all be different and the MK1 Cosworth rear ARB has a different mounting location to MK2's, but I should think changing the beam may negate this, also the front crossmember is different I should think.

Not sure if the hubs will be a straight swap, in any case the 3DR Cosworths are different to the Saphs anyway.

I should think the engine/gearbox should be a fairly easy swap with the correct mounts etc.

This is just the start, you then have the brakes/suspension, prop shaft drive shafts etc, ECU wiring etc.

Martin