Hello Bret,
I take your point about power, and although the O\D can be beefed up I don't know what it would be like with what you are anticipating. (I do have the same A type O\D on my Jaguar 3.8 saloon
with about 220 bhp, but it is only for 4th gear. You could do the same for your set up). The O|D box is the same lenght as non O\D but a bit heavier of course.
Just a thought, if there are any around, TR7 5 speed box, these were used in TR7's, TR7 V8, Rover V8 saloon (sorry sedan) and the Jaguar so it should be strong enough. With luck all you may need to do is fabricate a new backplate for your
engine.
Alec