If you don't already have one,get a good timing light with the dial back features.If you learn how to use one,your engine can be used as a distr. machine and really speed up the tuning process.Typically these engines like a total timing of 32-35 degrees all in by about 3,000,vac advance disconnected.Now,since you say that this mostly started with the ignition change.Where did you get that HEI from,and who made it.A lot of the lower priced units are made in China,and among other things ,have issues with electronics.As you may know,heat and electronics are not exactly friends.I'd leave the carb alone for now and dial in the ignition.