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

Replace the pistons with some good KB hyper pistons spec'ed out for the earlier engine. What mopar did was move the wrist pin up on the '72 340 and that made the pistons sit lower in the bore resulting in lower compression and less power. The factory 340 pistons are EXTEMELY HEAVY! I can not emphasize the enough. Get the good light weight hypers from KB. That will make a huge difference in how it runs. Light pistons rev faster and take a load of stress off the crank and mains.

Some Scat I-beams are very nice, way stronger then stock, and a lot lighter too.

You will need to have the crank balanced for the lighter parts.

If you can swing a 4" stroker kit now would be the time.

The '72 also had the smaller intake valve. Have the intake seat in the head opened up to 2.02" inch like the early heads and do a bowl blend.