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

The cast crank can be made stronger, and hold up for a while........But i wouldn't do it. If your going to stick with a stock exh manifold, you will be, most likely, safe with a Cast Crank.

Stroker Crank(4") or wait for some one, to us a stroker crank and buy there steel crank from them.

There just isn't a Half Way Decent stock cast iron exhaust manifold.

I Payed big money for a "Hooker Super Comps" on my E body, some 10+ years ago, steal leak free...........Well except for and occasional collector gasket.
I will NEVER buy another $99-130 header, EVER again.:read2: