sometimes I try to give more info than I need to. why do they make 3", 4",5",6" and 7" plenum depth manifolds ? from the shortest to the tallest , what if any effect will it have on the rpm range for torque and ,or horsepower ? I am well aware of advancing the cam. I am curious about how great the effect of plenum depth is in general. all open plenum ......
Sometimes it rules, sometimes it's just being able to fit something under the hood that gets different plenum volumes.
In my experience, adding volume to the plenum gained bottom without a notice drop in upper RPM power, to the point of making it taller just made it taller.
It's been years since I tested spacers like that. Some of what I've found is most intakes like the bottom of the carb farther away from the floor of the manifold than what comes stock on them. Again, that's almost always a hood clearance issue.
The closer the carb is to the bottom of the intake, the harder it is for the air/fuel to make the turn into the port. At low speeds this is much worse. The air can make the turn, but the fuel can't and it ends up hitting the floor of the intake. Once the fuel falls out of the air, it's damn hard to get it back into it.
Many low speed fuel distribution issues can be helped with just a simple 1 inch open hole spacer.
Again, before you lower the carb, I'd look for other ways to get what you want.
I have access to a flow bench with a fixture that is adjustable so you can raise or lower the floor of the plenum on an intake manifold and test what happens. Interesting testing.
