Chevy that thing should have no problem getting past 4500. Our 305 sprint car motor is 60 over zero decked with flat tops and stock heads as we can't touch em but they are shaved to the max,we did put smaller light wieght valves to help unshroud them,550 lift cam and roller rockers. In 2007 we ran it with a 650 alky carb,although the carb calculator called for under 600cfm don't remember exactley but 570cfm comes to mind. That thing pulled very hard all the way to 7500 rpm never pushed it past that, but I know it will. So I don't think your carb or heads are the problem...