I have read where people used 904 'guts' in a 727 case. I have also read that you can use a 727 valve body in a 904. It would be interesting to see a complete list of parts that can interchange, but that's not the point of this thread.
I bought an ATSG service manual that does cover both 904 and 727 off ebay for $15. I guess it wouldn't hurt to go ahead and pick up the 727 Handbook by Munroe, it's only $11 from Amazon.com. Anybody know of any other printed reference materials or maybe a video?
Here are a couple more websites with some good info.
A999 rebuild article - I wish the author would finish it. Some great photos, and there is a tip on how to remove the pump without a slide-hammer.