Rear diffs and gearboxes

As others have said, cut and shut driveshafts will be much easier than trying to get the imp shafts to fit the diff.

With losses through rear diffs, they use a spiral bevel gear (well technically a hypoid gear) to transmit the drive quietly. These have quite high frictional losses as the teeth slide over each other as they turn. My understanding is that the losses are increased as the teeth increase in size, as they're sliding further, so a diff designed not to take much torque can have nice small teeth, that basically just touch at a small point, with minimal slip, and minimal losses, but as you want more power transmission, you need bigger teeth, with bigger contact areas, so you get more friction as a result. I'm not really sure of the differences you'll find with normal production diffs.

The thing to remember is that some diffs may be both weak and inefficient, so don't assume that you just want the diff which is closest to your needs in terms of power, as there's bound to be some that are just a bit useless, so brake and have high losses.

1.6 Mk1 MX-5's might not be a bad one, they don't seem to have bad transmission losses, they're an alloy casing, and LSD's are very readily available. If I remember right they're bolt on drive shafts too, which can be nice in conversions.