Drivetrain advice for engine swap

Hey, guys... I have had my Dart Sport for 11 years and I am ready to swap out the 318 for something with a bit more muscle.

I have a few basic questions that may reveal some ignorance on my part so any advice would be appreciated.

Right now it has the 7 1/4" rear and the 904 trans...which probably wouldn't live long behind a 360 crate motor...

I think I have found a source for hopped up 904s that will handle 500 hp. so that seems to be do-able but the rear end still has me puzzled.

What is the usual route that people go when upgrading the rear? 8 1/4" rears are nowhere to be found around where... and it seems that 8 3/4" rear axle assemblies are somewhat easy to find but they're pretty much always from B and E bodies which are obvioulsy to long (wide?) for the A-body.

Any sources for new rear assemblies (case/axles/chuck/everything?)

Thanks so much for any help!!!