904 vs. 727
I use a 904 in my car and it runs 10.60's, maybe 50's in good air this year. I've cracked one converter snout and had the input shaft splines where it presses together with the rear clutch drum loosen up. I'm not sure if it was the input shaft or the drums fault but since I've replaced them both I haven't had another problem.
I have a couple friends that race big blocks in stock and super stock, they both use $5000 trannys that are 727 cases with 904 internals in them. Pro Trans I believe makes them. Those guys have to have the stock trans case but they can do anything to the inside they want. They are said to be worth about 2 tenths to them. The SS Hemi racers say the 904 guts are good to about 850 HP, after that they don't stay together.