vacuum gauge to solve idle issue

This seems to be the year issues with the cuda. Recently developed a stumble idle issue. Surging rpm at idle, also wont idle below 1000 rpm.
Have to shift to neutral at a stop and feather the fuel.
My initial timing is around 11* btc BUT thats about 900-1000 rpm.

I checked for vacuum leaks with propane and small rubber hose and couldnt find any(no rpm Iincrease)
I also have a mighty vac and checked vac adv unit, it held vacuum so I assume good.

I have a vac gauge and was going to use it to help diagnose issue(s).
I can't remember which port on carb to conconnect to??
Im not sure whether its vacuum leak, fuel issue or timing issue.
273 4 bl eddy 1405 LD4B, 904 trans.

Thanks all