Do you have the AFR readouts for the pulls?
Is the hole when acclerating from a stop sign or low speed part throttle?
Or is the hole when you hit a hill or go to passing lane on the interstate?
That demonstrates that light throttle cruise is mostly on the 'idle circuit'. A name that is certainly is misleading.
It also shows that your combination will tolerate the leaner high speed cruise provided by the 70 PMJ (primary main jets).
What flavor 750 is this Holley? An old school 4779 or something else?
Most likely something in the idle circuit. In particular the combination of idle restriction, idle air bleed and transfer slot exposure. That stuff OMM posted above. I don't agree on opening the idle air bleeds just yet, but that very well may end up being what you do.
Do check the transfer slot openings. I disagree with the set the primaries square as a goal. Rather set the primaries so they are roughly square, but an old school Holley can work from .020 to .040 showing. Others with different slot witdths and lengths will differ. First see where you are, then experiment with slight changes within that range.
It's the combination of IFR and IAB and transfer slot restriction (which is a variable airbleed and fuel-air restriction depending on the throttle blade position).
I think you can assume your gage is ballpark correct until shown otherwise.
Aim for performance. Cruise as lean as it will tolerate. Jet for best mph or area under the curve after determining the AFR is flat through up to the shift point.
