It will cost you about hmmmm $20 to weld the centre diff, and make it RWD, it cost me nothing, and the swap takes about an hour..
the box strength isn't so much of an issue with it in RWD, i popped an output shaft, but that was due to the diff failing, big time, and locking up ont he casing meaning all the power in 3rd suddenly wanted to go somewher,e so the next weakest item broke..
that said i have been told of people slotting 25 boxes into a rex, and using an r33 diff in the rear, that will cost a bit, as you'll need to fabricate a fair bit of stuff, get an adaptor plate made to mate the box and motor together, and then get a custom clutch made up, as you'll need a custom flywheel to make it all work..
but, realistically, the stock gear will handle the task, r180 if you can afford it, but otherwise, it will hold up to the punishment, just don't drop second with 4 people in the car at motorvation, and you should avoid popping diffs
