Colo,
I've had similar issues since I installed a GM Performance 350/290HP in my '72 C20. After messing with timing for a while, (trying 8 degrees, 10 degrees and so on), I finally tried timing the engine using only a vacuum gauge, (forget the timing marks). Assuming all your vacuum leaks are corrected, simply disconnect the vacuum advance line from your distributor, plug the port at the carb, (no open ports), and connect a vacuum gauge to your manifold vacuum source. Adjust your carb idle screw, (mechanical not mixture), down to the lowest that your engine will idle, then advance your distributor to the highest/most stable vacuum reading on your gauge. If you experience pinging, retard the timing back slightly, (one or two inches on the vacuum gauge).
This method worked great for me!
See the attached link for some great Q-Jet advise. You can also search the authors name on the internet for other tips, (I'd say this guy is a Q-Jet guru)
http://www.scribd.com/doc/24720655/How-to-Tune-a-Q-Jet
Also attached is a good piece I found on Q-jet idle mixture adjustment.
Good luck!