I think you have the EGR version. Here's what Edelbrock says:
PERFORMER 351W EGR (idle-5500 rpm)
Designed for street Ford 351-Windsors with EGR. #3781 is stock replacement/street legal part for 351-W 2V V8s; 1972-87 and works with either the OEM 2V carb or an aftermarket EGR 4V carb for off-road by using Edelbrock #8053 4V EGR plate and compatible EGR valve and gasket.
And it looks like this, with the same hole yours has:
So you need the #8053 4V EGR plate. Or, some way to positively cover that hole.