I have actually done a whole bunch of research into building a 904 for my own shop R&D race car. I never knew there were 2 different front servo sizes. The early small servo "up to 77 I think" will not hold a whole lot of power because it can't mechanically create enough pressure on the front band to hold it. I learned a trick with the front pumps to practically eliminate converter hub breakage. Use an A500 pump along with a custom converter. The A999 is the Trans to build if you can find one. Info is available but unlike a 727 is not all in one place. I don't want to get off topic on "sixpackgut's" post but things that give a 904 a bad name can be prevented and built into a fairly stout race Trans.
I am no trans expert, but I have raced 904's A500's and a ton of 727's in my cars.....The 904 is tricky to get it to live in a mild bracket car, let alone one as quick as yours. The A500 pump, and the A999, I believem have a hollow input shaft to feed the lockup convertor. This is a very weak area, and I would not put a 9s capable power train in front of one of those. The lockup convertors max out a 3000 stall, they are very heavy, great for cruising, but not the best race convertor. If you can get the 904 to live, it will be the fastest and more reliable than a lockup Mopar trans.
I know they take about half the amount of Hp to spin a 904 vs a 727, so put a deep pan on it, some good internals, and see if it holds up....
