this is what i did on my 440 . Was just a quick made stuffer to reduce intake volume , removed about 20% . Might have helped to have used a "cross" shape and fill area between front and rear runners as well , like where you have carboard . Helped a bit but as i have said before , i feel these are more friendly with bigger cubes and or higher RPM . I was only shifting @ 6000 trapping @ 61/6200 . Felt great on street , ran similar or hair more MPH but was lazy sixty footbraking . I spent quite a few pulls on a chassis dyno with it . Tried different spacers etc .
Tex
That thing in the floor needs to be sealed to the floor or solid to have any effect on the plenum volume, if it is hollow then the space under it will still fill and empty as pressure rises and decreases. I think it may change fuel flow through there or slightly change cross talk from cylinders on opposite sides of the engine but without being sealed it will do very little.
Also if the engine is going to be MPFI I would not lose any sleep over plenum volume but I would be trying to lengthen then runner length some. There is actually mathematical formulas to figure out ideal length and size rather than just adding some length and hoping it helps butt I hate math so someone else or a computer program would have to help.
