What carb?

Vacuum secondaries are always more forgiving. Double pumpers w/ mechanical secondaries will out-perform, provided you put the time into tuning it properly. You can't just slap one on and expect it to perform it's best on your setup. According to Holley's cfm chart, the best you expect your setup to flow is around 800 cfm at 8000 rpm, so I think a 750 would be a good start.