I'm not gonna guess on carb size, because the best way to find out is with the vac method that has already been mentioned, or on a dyno. I found out that my 600 cfm was a good size once I put the right jets in it. I think the dyno is the best way to tune it, because you can check the air/fuel ratio for the whole run and at idle. But I had a problem with my car breaking up too and it turned out to be my ignition system, and my shift light. My total timing was too high so it was breaking up, but I also found out that when my shift light kicked on it pulled enough current you make it miss for a split second, but that pulled out 25 horses and ft/lbs for a few hundred rpm. They are well worth it.
Phantom
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