Are both your e30's M10 models? You're looking at a lot of work to do this, but it's possible. IMHO I'd stick with the M10 and add a turbo, I'll bet it's easier. If you're serious, I'd locate an e30 M42 parts car before you start.
I just sent a late-model e30 wiring harness off to a guy doing the same thing - check out
Irish44j's rally conversion here.As far as mounting the motor...the M44 block is the same as the M42, so you'd need M42 motor mounts and arms. Front subframe is the same between all e30 models. Grab his steering rack while you're at it...the e36 rack is quicker than the e30 unit, only needs some spacers.
Tranny is the same basic G240 unit, but the earlier gearbox from the M10 uses a smaller pilot bearing. You'd need the M42 version.
You'll have to swap out the DISA intake manifold as I've heard it won't clear the hood. The e30 mani is the ticket. You'll need the M42 exhaust front to back.
Electrically, you're in for a real headache. The early e30 wiring is somewhat different than the later model. Worse, the engine harness uses the early square plug - you'll need a later round plug (C101 connector). You'd need to convert to a COP kit or figure a mount for the coils.
You might be able to adapt the radiator, but the later unit has an integrated expansion tank and slightly different plumbing.
The list goes on a bit, doesn't it? Not for the faint of heart, but do-able with determination...