Call Lenco Equipment located here in San Diego California
Lenco can answer your questions.
TurboHydramatic 400 is one of the heaviest duty automatic transmissions ever made.An excellent starting point if the right recipe of parts is selected for the intended power level.
After 450 horse is exceeded and high shock loads are gonna be common the sprag,input shaft ,pump, thrusts, servo,planetary,sets all have to be upgraded to be able to live behind HP/HT drag race type applications.
You can even get lower geared planetary sets for mor torque multiplication in first and second gear.Third will always be a 1:1
Coan has a lot of parts for this.
The 400 will require at least annual or sooner maintenance depending on power levels and # of passes per season.
Just my opinion here:
Most all powerglides used in ultra highpower applications are all aftermarket clones now.Original powerglide cases are not preferred anymore in those types of applications.Same goes for most of the internals.Just like top fuel hybrid hemi clone engines they are now purpose built racing transmissions with cases that share the original design concept and exterior dimensions.
The 'glides and 2 speed Lenco drives are preferred in light weight high power cars ,altereds and dragsters before you get to the direct drive slipper clutch power levels.
Over 3000 lbs,depending on the power level ,you will probably want 3 gears especially in the 1/4 mile.
If you were exclusively 1/8 mile a powerglide might be the ticket with 4.88 or lower (higher numerical ratio) gears.