904 Parts

I just rebuilt my first 904, it's pretty straight forward. Get an ATSG manual, go to YouTube and watch the 19 part series for 727 rebuild, the 727 is almost identical having larger parts. In YouTube type in "rebuilding 727 torqueflite 600hp". There are also some other little short videos found when you type in 904 torqueflite. Having a shop press will be handy for rebuilding front clutch pack and replacing bushings. And air compressor for testing clutch packs and servos.
Use search function here, a lot of the questions you will have are here as well.
For parts many of the American guys have some favourite places to buy, in Canada I used TCS for parts(which also does a lot of American sales). I bought my servo/accumulator upgrades through Makco.
My 340 going in front of my 904 is probably gonna make about 400hp, I'm guessing you'll be in the same range. I went with 24 return springs module for front drum, and the other basic upgrades like high friction clutches, kolene steels and HD bands. I left mine as is with 4 clutches each front/rear. I replaced bushing and put a new gear in pump. Replaced springs and rollers in the one way sprague. My valve body is a full manual, I prefer to shift my autos manually then mess around with kickdown linkages, plus I like shifting. This 904 is forward pattern and my Demon's 727 is reverse, I wonder how often ill mess that up, doh.
Just watch videos and read manuals and other threads here. I also have the HPBooks Torqueflite A-727 Transmission Handbook, which has been handy too.
Good luck with the rebuild.