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

70's had bigger intake valves (2.02 vs. 1.92). They also had higher compression. they are rated at 10.5 to 1 (actually they are lower than rated, usually only about 10 to 1) where-as the 72 is 8.5 to 1 (also probably a little lower than rated). The diff. is in the piston height cause the heads have the same chamber size. Cam is possibly different also but your gonna need a much stouter cam than even the stock 68 4 spd. cam to even make 400 hp. Install 2.02 valves in your J heads and do some port clean up and a perf. valve job. Install higher compression pistons (shoot for 10.25 to 1) and balance the assy. Install a bigger cam, Edelbrock Air-gap intake, 700-750 Holley, headers and your in business with at least 400 hp.

A buddy of mine had basically that same build with a comp solid flat tappet cam (20-231-4) and even though he never dyno'ed it I'm sure it had to be in the 450 hp range as it ran low 12's with just an average tune up. I think if he would have ever dyno tuned it it would have been solidly in the 11's.