I have had at least ten off. This is including 1st gen Camaros. The way I always do it is to use the 'F' dimension from the service manual as a front to back location. Then I find the centerline of the body and the centerline of the subframe. I line up the two centerlines and check dimension 'F' and lock it down.

For a clearance reference, I get about 3/8" to 1/2" clearance from an HEI to the firewall. This is the same for 67-69.