Best replacement pistons for stock '69 engine

Those pistons should work fine for a relatively stock application. With the stock cast iron heads (675 casting) I'd keep the compression ratio at 9:1 max unless you want to pay more for higher octane fuel. If you're adding a 4bbl and headers, I think I'd get a slightly higher performance cam. Something around the specs of the 340 purple shaft seems to work good on those 318 4bbl setups. You can go slightly less if you like a slightly smoother idle, but, you need one that will take advantage of the 4bbl and headers. Otherwise, why bother doing the upgrade, just leave it a 2bbl.