Re: SBC Running Rough
As Dave said, initial ignition timing around 10 BTDC plus vacuum advance is important for idle quality.
Rough idling after replacing the heads could be due to setting valve lash too tight, which would keep valves from seating and mimic a vacuum leak. Hydraulic lifters work best with between 1/4 to 1 turn down from zero lash at the rocker arm.
I prefer to adjust the valves with the engine running. I use an old valve cover with the center cut out to temporarily replace the usual rocker arm cover. This give access to the valves while keeping most of the oil in the engine and off the exhaust. Back off one rocker arm nut until the rocker begins to make a clacking noise for at least five seconds to allow the hydraulic lifter to fully pump up. Then turn down the rocker arm nut until the clack noise stops, then another quarter-turn down. Adjust each rocker arm the same way.
1970 Chevelle Malibu purchased new in 1970
355 SBC, XE262H cam, Performer RPM heads, MSD HEI, TH200-4R trans, 3.55:1 Posi, digital gauges, QA1 coilovers