would a twin turbo 20b fit into an rx-8? if so what would need to be done and how much would it cost? how difficult would it be?

Yes it is. I'm not sure on the details, but there are a few out there. the front clip is almost 5k all together.

I read somewere that rod millen's drift car has a 20b.

Here you go:

http://www.kiwi-re.com/gallery_29.php

it's the 6.81 sec 1/4mile 20b'd rx8