13:1 Compression Pistons in 340 Small Block

To the OP, I would take a step back and decide on a clear goal for the car (horse power number or ET goal, or drivability goal). From what I've read, it sounds like you want to use the domed "13:1" pistons, want 8mpg or better, want to beat on big blocks, want to use J heads....These goals don't really go together. Those heads won't support big power (no big block killer), those pistons belong with fully ported aftermarket heads (no J heads), and 8mpg won't happen if you have the engine make big power (generally, the bigger the power, the lower the mpg)...does that make sense?

If your goal is a fast street/strip car, just remember it takes way more work (and $$$) to make a car walk that thin line of race and street.