Get a 71-up 904 case. They have the bigger diameter servos.

Pre-68 have the wrong input spline count for RACE converters.

I think........76 and later 904s have the steel planetaries for
the 2.45:1 1st gear or get a 2.74:1 1st gear-set and swap.

Valve body from 727 is the same.

Multiple return-spring conversion for high gear drum is a must.

Longer driveshaft is required due to the 904s 3.900" shorter OAL.

1350 slip shafts are made by a few.

I'm no 727/904 specialist but, these are a few things I've found during my switch.
I hope it helps a little.