Depressing Dyno results. Need advise

Which dual plane intake, Performer or RPM? And just how mild is your cam?

If you have a fuel delivery problem, and it sounds like you may, fix that before you go swapping carbs. A 750 should be OK for at least 100 more HP than you have now. And, tuned correctly a vac secondary carb will make the same HP as a double pumper (but Holly Double Pumper sounds more like a **** star).