727 vs 904?

Trans parts and converters all being equal, a 904 will be about .15 faster in the 1/4 mile, has a 2.71 (or 2.74?) low gear option and is less prone to grenading.

A 727 is naturally stronger and will likely be good for most applications.

Both started appearing with lock-up converters in the late '70's, which is not the best choice for performance builds. Many parts, including converters, can't be "mixed". Either all lock-up or non-lock-up. But the lock up parts can be swapped out as a set.