Assuming you have a Q code 351 Cleveland like mine, let me make a suggestion or two.
Check the points in your distributor and check that the vacuum advance works freely. (unless the distributor has been converted, then just check that the advance mechanism is functioning properly.)
Check the cap for a crack or corroded terminals and check your wires for shorts. (Mine looked good, but would shock you in a heartbeat if you touched them when I got my car)
You can also test the coil to see if it is breaking down when hot.
I haven't mentioned checking the timing, but it isn't a bad idea.
My car had a 700 CFM Holley 4150 when I got it in basically stock trim with a 4 speed. That was plenty of carb, now with closed chamber heads, a big solid cam, headers, very free flowing exhaust and a 5 speed-it is still plenty of carb for spin up to 7,000 rpm (though the dual points were only good for about 6500)
How is she running on the 600? If she runs well on the 600 with no back firing or other bad habits, you may have a warped body in the 750, or some other damage.
Cleveland head ports are huge-as big or bigger than big block heads and they spin up higher RPM than most stock small blocks. If the rear end ratio is 3.5 or lower or the car is a 4 speed, it may very well need a carburetor that is larger than conventional knowledge suggests. Then again, that 600 might be just about right for a stock cammed automatic car with 3.50 or higher (numerically lower) gears