In neutral or when driving?
With factory cam should not be rough in either case, but even with cam, should be smooth in neutral.
Both the original '68 and the '71 carbs have a relatively lean idle. Lean fuel mix relative to pre-emissions control. So the engines were set up for slightly higher idle speeds. You might have to idle it closer to 750 rpm than 600 rpm.
Backfire through the carb can only happen if the mixture is igniting before the intake valve closes. Check the initial timing and then set it a two degrees less advance. See if that helps. If the distributor is original or correct, you can use the factory timing specifications. If its a generic replacement the timing increase for every few hundred rpm should be measured. After you plot the advance curve, then you decide on the initial.
