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

Headers will always be better but the HP loss can change depending on build.

For example…… My build. High compression, high RPM. I went form 11.78 @ 118mph to a 13.2 @ 105mph most (but not all) of that is related to exhaust changes.

My engine is built for a shift point up around 8,000rpm which was fine with Hooker super comps and 3" exhaust.

Basically I lost 1.42 seconds and 13mph when I went to 340 manifolds and 2.5" exhaust.