The only problem with too large a carb on the street is that generally with an OOTB holley 4150 is you need roughly 20-ish % of rated airflow to initiate main metering, a good 650 will make more torque at part throttle and the transition will be less of a 'light switch' power-wise. A pro-form CNC replacement body on a 650-700 baseplate would be a nice upgrade. Plenty of 11 and 12 second cars run 650's, Really a 340 only needs about 600CFM of total airflow to turn 6200 RPM assuming 100% VE, the smallish .480 lift cam probably won't pull it much higher than that and it'll be past it's power peak sooner than that anyway (so the VE will be falling off). A smaller carb will also make a slightly loose converter a little better part throttle drivability too...opinions vary I guess but I've been running 340's on the street since at least the early 80's (including a 4 speed W2 headed 69 dart in 80-81).

The RPM and Air Gap are functionally identical, once the motor is warm (from the coolant flowing to it and around it and the crossover heat on the flanges) the manifolds both heat up about the same (IMHO) in normal summer driving. If it was a bracket car, I'd run the Air gap just so I could pack some ice under the plenum between rounds. t oput it another way...I wouldn't pass up a good deal on and RPM to hold out for an Air Gap on a street driven car.