The 323 GTX rearend would be perfect, if you can grab just that. They are super rare though. Not sure how well it would work with 100% engine power through it, but it'd be a good start
The Mazda AWD transmissions are made of glass. They're not fun, because you can't abuse them. That's what's wrong with my conversion. The center diffs are one of the weak points. Putting axle plugs in and sending 100% of the power thru it is a sure way to blow it up.
Why not do an AWD setup? It's way, way more viable than RWD. The Capri chassis is not made for RWD (even with reinforcements) it's going to handle like crap. You can go AWD, but pick a decent transmission. Toyota has an AWD version of the e153 that the Mazda guys swap to, with the assist of an adapter plate. I've also seen DSM swaps into the 323 chassis using the whole of the DSM drivetrain
If you've got the Miata, keep the bits in the miata. The chassis is designed from the ground up around the drivetrain, and that's what makes them good.