I'd stick a large displacement straight six in it, and maybe upgrade to a better overdrive transmission. The T5 will do good up to about 330 hp if they're built up strong. That should have most stovebolts covered.
If you want an automatic, I'd recommend a 700R4. They're nice transmissions, with a nice steep 3.06:1 first gear ratio.
After having converted mine to a V8, I can tell you, the conversion is no fun. Literally EVERYTHING has to be modified. It's such a tight squeeze. Besides, my cool factor went down when I took out the stovebolt engine.
