Are small block Edelbrock Performer heads wider than stock heads? Because Doug D453s with performer heads and a Borgeson box definitely don't bolt in place without beating on them with a BFH.
I tried using the thicker motor mounts and while they help, they only cause you to have to beat clearance into the headers a little less. But these motor mounts also cause problems such as your distributor plug wires hitting the bottom of the wiper motor and the pinion angle getting out of whack.
I'm currently working on getting them working with stock mounts and home made 1/8" motor mount shims.