Holleys are the best performance carb made, other than the Race Demons, which are Holley clones, but improved versions.
I'd expect more vacuum with a 230 dur camshaft. Unless you're only running 9.5:1 compression? Or you have the idle speed really low?
What idle rpm in neutral?
How much initial timing? How much total timing, and at what rpm does it come in?
What method did you use to adjust the idle mixture screws?
The cam isnt the only adjustment for the accelerator pump. There's pump squirter size, and also the position of the pump cam, too. (you can also flip the cam upside down for further adjustment possibilities)
What method did you use to adjust the accel. pump arm? (the locking nut) If the car is idling, and you wiggle the pump arm up and down, you should just have a tiny bit of lash. If it's too loose, you'll have pump squirter lag, if it's too tight, you'll get a reduced pump fuel shot, which may not be sufficient.
Lastly, a 750 Holley is somewhat too large for a 347" motor running relatively low compression and a
[email protected]" dur camshaft, because it doesnt have the RPM potential to take advantage of the big carb, especially on a dual plane intake. You can bandaid the slight overcarb condition by playing with the accelerator pump adjustments to get rid of the off idle flat spot caused by poor vac signal.
If you bought a new 650 or 700 BG Race Demon double pumper, the difference between that and a standard Holley carb is very noticeable. Even a Race Demon 750 would be fine, since those carbs are much more responsive to vacuum than the Holley carbs.
If you're determined to use that 750 Holley, you can still get it to run very nicely with some patience.