That's kinda what I was getting at. You defeat the purpose of using a drop base on an AFB by raising it up with a spacer. I think I have been there before, but couldn't remember for sure.
I have a Performer intake with an Edelbrock 600. I installed a 1 inch spacer so my breather clears the dizzy and using some clay I still had about 3/4 inch clearance when the hood was shut. Is the 351 really that much taller? What intake are you using? The Factory air cleaner is a drop base air cleaner.