What's the space specs on the 60's trucks versus the 80's trucks?

The mounts I got from BRP (and the way I mounted them) puts my 5.3 about 4" off the firewall in my '86. The reason I'm leaving this way is because I'd like to leave the 5.3 compressor in the stock location. If I switched the mounts to the opposite side & direction, the 5.3 would move back 2". As it stands I'll have 2" clearance (at the most) with e-fans.