Tranny Recommendations for 600 HP

What is your intent for the car? Drag only, street only, etc...
That said, personally I'd go with a 904. It takes less HP to power the tranny (I believe it has 15-20 HP while the 727 takes ~40). One other thing you might want to consider is the GM 200R4. It's all hyd o/d tranny that can take a LOT of HP when built and like 904 only takes ~20 HP to power it.