Super tuning car runs good shooting for great

The sparkport requires the throttle blades to pass by it to relay the vacuum to the Vcan. Then for a time, the signal strength will increase, and eventually it will decrease. It may or may not ever match intake vacuum exactly.
With the car in Neutra/Park and the gauge plumbed in, slowly rev up the engine to say 4500 and watch the gauge. You should see the vacuum rise, but you may not see it peak on account of it doesn't take a lot of throttle opening to hit 4500.
A better solution is to put it in 2nd or even third, and slowly accelerate the car.
In direct at 65 the throttle may not be open far enough to activate the port. I have had to lower the pick-up point on several carbs over the years,including a couple of Holleys.
The problem is often that the engine is too powerful, and/or the rear gears are too easy, and then the carb is barely open at cruising speed.