what needs to be changed in a 72 340 to be like a 70 340?

The differences? Carb & intake have minor differences. The crank is cast and exernaly balanced so that makes the balancers/damper/flywheel/flexplates for external type.
Pistons as said earlier. Heads have the 1.88 intake, not the 2.02.

To make it perform like an earlier 340, higher compresion pistons like the above said KB would be a good start. You'll need a new cam. Though all 340's have the same cam minus the one mentioned, the 68, 340 4 spd set up. You might as well start fresh on everything.

tti headers are the ones you want to make the HP you seek. Otherwise, the exhaust manifolds will hold you back 70+ HP.