I've noticed that while cruising at a constant speed the engine has a very slight hesitation or buck to it. If I press the throttle down just the slightest amount, it goes away. I did notice that after using all the Mobil gas in the tank and filling with Citgo gas it got much better.

Motor is a 350 sbc, 650 mighty demon carb, Canfield heads, 20 degrees initial timing, 35 total, 10 degrees vacuum advance. Primary jets are 72's. To cruise at 50 at 1500 rpm I'm barely on the throttle. Its little enough that how the idle is tuned affects how it drives.

you probably have too much timing advance. Mine did the same thing so i retarded the timing a few degrees and it went away

Thanks. I'll try taking 5 degrees of vacuum advance out of it.

How old are your plug wires? Make sure they are all good and not burnd or arcing to ground.

my car w/ a 6-speed does it too, your most likely just lugging the motor