[Formulajones]

Those dome lids pictured are what the roundy guys always run or anyone serious about making power, and Ive had them worth a little power on the dyno as they help direct air into the carb, that's the theory anyway.
Doubt you had a vent clearance problem as 3/4" is plenty, but any gain you might see is likely just from the lid design itself. The flat lids with sharp angles suck.

Same goes for the drop bases. The flat sharp angles suck. You'll want the smooth round drop base and they come in all depths. I typically like to use the OEM drop bases as they are round and have the most drop of any. They are repopped for the various cars they were equipped on like 67-69 Z/28's, L78's, etc...
Even the 67 GTO with the optional HO 400 that came with an open element likely had the same smooth round drop base from the factory.

Just throwing out food for thought