1973 340 detuned?

You are on the right track with going to the earlier piston - that should help a lot. As you are replacing pistons anyway, just go with a higher compression model piston. Someone has to make one. Check with Mancini or one of the other Mopar specialist shops. As far as the heads go, be aware that if you cut them too much, you will have manifold alignment issues. There is most assuredly horsepower gains in the heads, even just matching the ports to the gasket(s) - along with the intake ports will make a difference. Unshouding the valves and a decent "bowl" cleanup gives you even more! Since you have it apart anyway, check to make certain your block has a nice flat deck - and maybe give it a little .010" cleanup (that isn't a lot, but it all adds up). Cams. Wow. We have So much to choose from! Spend a little time talking with one of the guys from virtually Any of the cam makers out there, explain what you want to do with your car and above all, Listen! In the 60's I had a 273 Commando Barracuda that I recammed - I wanted that drive in idle, but the guys at Crower talked me into a much more suitable cam. Didn't sound bad azz, but it sure did work good! As far as timing goes, 32 to 34 degrees total seems to work well.
Just my ten cents worth. (inflation, ya know!)
Have Fun! Art (R/T)
65 S 331LA, 904/998 & 65 318(soon to be 340) 4speed Barracudas