The quest for 400 HP is easy. linked below. As you're pointing out, it's the configuring the rest of the vehicle, with the electronic trans to hold up, and work with it.
https://blueprintengines.com/collections/gm-compatible-small-block-crate-engines/horsepower_401-500
if you go carbed, you'll need a stand alone trans controller.
if you re-use the factory TBI, you're asking alot of it to feed 400 HP. even after a tune. those flow around 550 cfm no?
if you go with one of our versions with aftermarket EFI, those can sometimes be tied into the existing GM setup to give the trans the TPS and such it needs to operate.
we can make anything work, you'll just find your next weak link in a hurry. don't forget a higher stall converter when you're beefing up that trans.