1970 340 heads on a 73 318

If you are running factory manifolds, with no plans to change even to HP manifolds (the 68-71 340 HP manifolds..), I'd leave the heads alone. A cam change, will accomplish what you want, without causing more issues. The head swap, is a double edged sword. You get better intake ports, bigger exh ports, and larger valves, all of which help in power production at higher operating ranges , especially ona small displacement engine. You loose port velocity on both int and exh, and static compression. Both of which lend themselves to power production at lower rpm ranges where stock gears, convertor, and manifolding dictate the egine run at. You cant ram air thru those log manifolds. So, any extra flow cant get anywhere easily. And, you'll loose idle quality and response becuase of the compression loss. Compression enhances power from 200rpm up to redline. any loss there is significant. My suggestion, would be to install a small hydraulic cam in the 318, as the current one isnt even designed for a 4bbl engine, much less the 318 ports (which really are not that bad for a small displacement engine). Any new cam should be degreed during installation, so you really get what you expect form it. MP cams especially..