A 904 in a lightish car of say 3000-3200lbs will handle 500ft/lbs with most of the standard hard parts.
904's don't have the friction cross section that a 727, Powerglide or TH400 has, so they don't have the torque handling capability.

A Proflite is a different trans to a 904. It's a 727 AMC case, with custom drums etc and uses a Powerglide converter.

There isn't an aftermarket case for the 904 or 727, so the standard case is what you're stuck with.

I maxed out 904 is going to set you back around $5k in parts and is going to require a lot of maintenance. Most guys that run them always have a spare.