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.