408 possible timing issue

Hey all I have a strange issue that I cant figure out. First off the car is a 73 dart with a 408, mp electronic ignition and a mild cam and 9.8 to 1 compression. Now for the problem, when in neutral I can rev to the moon, but when I start driving it will only rev to about 2400rpms then it starts to feel like its leaning out. I've bumped the jets up three sizes, changed out the dizzy 3 times, changed the ignition box twice, changed plugs and wires and have had the timing set from 5 btdc to 15btdc with full advance coming in at 2500 all the way up to 4000. The carb is a holley street avenger 750 vac adv, sitting on an edelbrock performer rpm air gap manifold. Any help would be greatly appreciated!