Check the throttle shaft for play/vacuum leak. While its running at idle try to wiggle it and see if the idle changes. I have found this to be a problem on many 1920's
I would recommend you run the valves if you havent already. Ive seen it make quite a difference in idle quality, especially if a po didnt do it correctly prior. You should also check for air leaks around the throttle shaft of that 1920. Good luck.
X2 to the above. I would stay away from the spacers/insulators that are stacked gaskets and aluminium plates. They can lead to cracked baseplates if you dont pay careful attention to your tightening sequence/torque.
What you need is a paper template from a member that has a trunk lid w/out the emblems on it. It would be much easier than trying to figure measurements. Hopefully someone will be along to help you out. Good luck
Ive seen it on several GM motors. Most of the time it slips just enough to set the timing mark off. Balancers are one of those things that are often overlooked and taken for granted on a build. Definatly worth giving it a good cleaning and inspection for cracked elastomer or heavy seal area...
Just my 2 cents on going to a bigger/better cam (great idea by the way)...you may want to spend the money you have for the converter on porting those Eddys instead. That way you can really take advantage of the bigger cam...just something to consider.
This is good advice from flyfish. You...