I was told years ago(mid 1980s) that the 1966 B body trannys had the best cases to use for drag racing by Dave Smith at Pro Trans.
I'm not sure what year Mopar started adding the 4 pinion planetary in the motor homes and H.D tranny for pickups and Vans shruggy
600 HP is not a lot for most 727 with good parts in them twocents
I use to build my own drag racing 727 and then figure out is was better for me to Have Dave at Pro Trans build mine form then on work
I've had tranny built by A1,Art Carr and several other SO CA drag race tranny companies before switching to Pro Trans. I recommend you contact them scope
Their located in mid Texas now so shipping is faster and sometimes cheaper than when they where in SO CA scope
Their are somethings I want done for me by the best in the business like brain surgery, race trannys and several other things I won't bore you with on here now work

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)