i need some engine advice

66fs said
I'd super tune it first. Timing, total advance, good TQ carb set up just right. Should have "J" heads on it already, should have a premium forged crank, but maybe not. Should have the good 340 cam and springs. You can not run 68 to 71 pistons on pump gas with the stock cam anyway. If you get it tuned perfectly, maybe some gears. 3.55 or 3.91 with a sure-grip. Enjoy! If that is not enough for you then as Moper says...

A '72 340 most likely has a cast crank instead of forged. It's possible it had a forged crank, but the cast is more likely.

You can definitely run 68-71 340 pistons with pump gas and the stock cam with most stock engines. The factory spec'ed 10.5:1, but the compression was rarely that high. The compression on my '68 340 was closer to 9.5:1 stock based on the chamber volume of the heads, it would run 91 octane no problem.