598 cubes, 8.6 to 1, What plug gap should I run with MSD ignition ( 6M )

.035 would be fine.

The same as a any Cheby back in the day, funny how things stay the same as things change........035 is a universal gap, Chevy, Brigs&Straton lol

I like .035.
More gap will be harder on everything.

I like .040 with msd

Everyone is correct (IMHO and experience) above.

Save bigger gaps for the beach. Hah !

Gap depends on what heat range plug you are using.

Back in the day, if you had the "High energy ignition", you gapped the plugs at .060. If you didn't have the HEI, with a conventional ignition, you gapped them at .035 on most GM V8's.

I had several pontiacs, with factory HEI's. .060 was the recommended gap, never had an issue.

.035-.045 imo, is just fine for what you're doing. I've ran my plugs at both of those gaps, never seen an issue, or a difference either way, in my boat engines.

I agree if you have a large distributor cap you can go big gap but most guys here only run a 3" distributor cap no good for big gaps.

msd recomends 55 -60 for up to 10.5 c/r.35 to 40 will work every time up to 15 lbs boost.really no reason to widen the gap imo.