Peak power in your 400 with that cam is going to happen before 6000 rpm. Your thinking you're losing power by tightening the plug gap is likely all in your mind, but it looks like you've found your problem. I've never run a gap larger than .045" with an HEI (with or without an MSD box) because based on timeslip results, it didn't make a difference with a larger gap, so why tax the ignition system? Sounds like a bad coil (or bad ground to coil).
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