Et and speed difference with a good converter to a mismatched one can be quite a lot. For instance, my dad's chevy nova with a small block and nitrous was over carbed with , only a 3500 stall and 3.5" exhaust. Went high 12's on motor and 11's on nitrous. Lazy dog off the line. Single 830 carb, 5500 stall converter, no change to exhaust, would leave on the wheelie bars with a 1.54 60 ft. Ran then 11.10 @ 120 on motor, 10.35 @ 130 on nitrous. The combination was way off originally. You have a ton of potential with those heads, but everything in your combo must agree with each other