Big Cam (to me) and Low Vacuum - Is this Correct?

[66Satellite47]

The drop in AFR when "stomping on it" is also a function of the accelerator pump, which may require tuning.
Without seeing the AFR meter & vacuum gauge side by side, I can't really comment much.
Where is the vacuum gauge connected?

 

[Wietse]

The short, instant but temporary change in the AFR reading is all about the accelerator pump shot, this is purely to compensate for the fuel that "drops" out of the air when the manifold vacuum falls.

You can test by installing a blank PV and test run the car, while increasing throttle observe the vacuum, once the main jets cannot longer provide sufficient fuel that is the point where the power valve should be activated already to enrich the mixture.
At that point the AFR meter will go very lean and the engine will have power loss, say at 8" Hg manifold vacuum for example.
In that case a 9.0-9.5 PV will suffice.
You could even play with it by installing a smaller size primary jet if the part throttle AFR is on the rich side and install an even higher PV like 10.0-10.5 to help fuel economy.
But that needs to go by trial and error and record all changes and effects as you go and see what works out best.
It is not as easy and straight forward as most say with good vacuum en smaller cams.
I have tried and tested loads of things had good improvements with the help of AJ who is a member here and on FABO and knows a ton about this.