Got it scoped. You need the GP adjustable mount, which has plenty of "meat" to move the mounting holes if necessary. The Sig mounts are smaller along the mounting beam, which is why I wanted to use them; those GP mounts have a lot of unecessary bulk. I have one set up for a 61 FSR, which has the same "footprint" as the 91 Surpass. It fits the AeroMaster firewall fine, albeit with some overhang off the top of the firewall. No worries; there is some additional structure up there to cover it. Just hog out the mounting holes to move the upper bolts down a tad...

Now, to get a new mount for the 91 Surpass. Usually, the LHS has a wall full of these. Naturally, they are out of of the 60-90 size...