Holley 600 carb help

If you need to buy another pump diaphram, get the green GFLT diaphram. It's not as prone to cracking from the ethanol in our fuel now. Cars that sit with those black diaphrams have them get hard, shrink and crack.

The one you bought is correct. The old one has shrunk.

The hesitation may be from the accel pump.

I don't run ANY mopar with only 10* initial timing. Where is your total number now and does it ping at all? Get that total number, dial in the initial to get that correct and fix the curve. It may require you limiting mechanical in the distributor to hit your total number. I'd pump the initial up, it will idle cleaner and may get rid of the hesitation. If it's got any kind of aftermarket cam, at least 16 initial would be my suggestion.