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depends on cam, motor and usage, but most likey 4 degrees advance would be ok,moves peak tq and power arond 400 to 500 rpm lower and lowers peak horsepower.
no headers, does it make that big of a difference? i cant say for certain its a 390 i know it came with a 360 but the guy i bought it from said a rebuilt 390 was put in a few years back.
Headers make a big difference in power output. Any FE performance build requires them.
If you really just want a low-rev puller, they aren't that critical, but you should do the dual exhaust, crossover pipe etc or equivalent larger pipe to eliminate as much restriction in the exhaust as possible.
it already has a dual exhaust on it but each side is isolated, does the h-pipe make that much difference? ill measure the stroke best i can through one of the plug holes today.