gap for plugs - MSD box & dist.

In all my years of running and racking my 340, 383, and 426 Hemi, I never found any major difference between new plugs. As Rick Ehrenberg said, ignition does not make more power as long as it is properly functioning. Timing and tuning are far more important (except for maybe heat range if far off).
What no one brought up is the need for vacuum advance on street engines. There is much to be gained with vacuum advance on the street, and nothing at the track. But if you want an engine to be economical and last, vacuum advance is needed. 50-52° at cruise vacuum is ideal. Adjusting to maximize cylinder pressure at part throttle without knocking is needed though for some engines.
Properly set up with a mild cam, a 340 can get 22-24 MPG with the right advance curve. I did that in the late 70's when gas lines were horrendous.