'81-up on the 302.  If it were an '80-'81 engine I'd examine ti real close because changeovers are never absolute. 

What TKO said…

The block has nothing to do with balance.
Pistons are same.
Rods are same.
Cranks are different. The 50 ounce cranks were lighter. 
Harmonic balancers are different
Flywheels/flexplates are different.

You can use a crank designed for a one piece rear man seal block in a 2-piece rear main block, but you cannot use a 2-piece rear main crank in a one-piece rear man block unless you machine off the flange in the seal area of the crank.

All the various parts like crank, harmonic balancer and flywheel/flexplate will bolt up to either type of balance, so be careful you match the crank, balancer and flywheel before just willy-nilly throwing an engine together.

A machine shop can do pretty much anything so long as you're willing to pay for it.  You can rebalance or even zero balance the assembly.  A zero balance on these engines is going to be very costly because you'll need a lot of Mallory metal to make up for the lack of ability to balance it normally (why they used an imbalance from Ford).  Most strokers for the 302 are actually 28 oz. imbalance, not 50oz.  Moving the rod throws outward from the crank centerline moves enough mass that they don't need as much imbalance. 

A zero balance sounds sexy, but in reality in a street engine, or even most low buck race engines for the weekend warrior the cost just isn't justified. 

As far as why Ford changed, my understanding is that it was cost driven.  They wanted to lighten the 5.0 cranks to save money and this left less mass to balance the engine in the crank, so they had to go heavier on the balancer and flywheel/flexplate.