I have built many 400 transmissions for high 7 second street cars, 600lbs-ft of torque is no big deal. Look for a 69-71 core. Those will have the good aluminum pistons, the updated center support, and the smooth style direct clutch that will accept a 34 element sprag. All those parts can be added, but with additional expense.
Use a TH350 pump bearing and shim in place of the rear selective thrust washer and shim. Get rid of all the plastic thrust washers, either buy a bearing kit, or make sure your core has the bearings with additional rollers.
Use a 4L80 intermediate clutch backing plate, 4L80 thin steels, and 4 standard 400 clutches. This will allow 4 clutches in place of the factory 3. Use a .106 snap ring from a 727 Chrysler rear clutch. It’s a little thicker, and much wider than the factory snap ring. This is the same snap ring included in the TransGo shift kits.
If your using a valve body with fixed line pressure (no vacuum modulator) make sure you use Teflon sealing tings over the cast iron rings. The high line pressure will eventually wear out the rings and cut grooves in the parts the rings seal against.
Don’t use the stamped steel pistons. They are pretty flimsy. Use the early aluminum pistons, or the bonded direct piston and return springs from the 4L80. The direct piston will fit the forward clutch, just omit the center seal. There are clutch kits to fit 6 clutches in a 5 clutch drum, or some of the later 4L80 direct drums have 6 clutches factory.
The forward clutch hub is probably the only questionable part. Use a aftermarket steel hub, or the 4L80 uses a steel part.
Im not going to offer suggestions on valve body yet, that depends on if you want a auto shift, manual, and if the manual is a forward or reverse shift pattern.
The 400 is very strong, properly built and with the tight parts it will hold up to more power than the engineers ever thought it would see. Just keep it clean, pay attention to end play, and work slowly and carefully.
