Vetteman, How thin is thin? I have a Corvette/Camaro LT-1 intake on my Chevelle's small block and I have a Moroso 4 hole phenolic spacer there for heat insulation and it is an inch thick. That helps add plenum volume. The 4 holes help with velocity. A more open one helps better with mid to upper rpms. If the car isn't run hard then a 1/4" thick "spacer" isn't gonna help much. It is most likely there for heat reasons. GM also made a heat plate for the Z-28 in the late 60s. Just get that and it'll keep the fuel bowls cooler.