tuning a magnum

I'm new to a-bodys only. I've searched high and low for good tuning info on magnums with little results. first off my car is a 67 barracuda fastback with 5.9 and 18 spline 4 speed. 391 gear. stock bottom end, r/t heads with 2.02/1.65 valves and hughes stage 2 porting. m1 intake, speed demon 650 vac secondary carb. hughescam .228/.236 duration @.050 .544/.549 lift with 1.6 roller rockers. fuel supplied by a malory comp110 efi pump, 1/2" supply and return lines, fuel pressure set to 6.5. Msd 6digital plus box, pro billit dist and blaster2 coil... with carb at factory specs i need 20* initial timing before i really stop gang vacuum(15"at idle) had to turn a bushing to limit the mech advance to 14 which gives me 34 total and its all in at 2300. Engine staeasy and idles great. Normal driving its suprisingly well manneed. but under full throttle it stumbles at higher rpms. off the line its fine or from a rolling start. it will pull hard up to maybe 4000 then acts like i hit a rev limter for a second then comes out of it and pulls again but as as hard as it should. I've never ussd a vac secondary carb before and I'm tempted to switch to a double pumper but I'm not convinced that my problem lies only in the carb. the secondary spring is gold colored which i believe is medium tension. Float levels are set halfway front and back. anybody out there running a similar combo with any advice would be greatly appreciated. i really wanted to run this year at the track in terre haute but now thier done for the season. i still have hopes on burning some tires before it gets to cold up here! Thanks in advance. cam