ORIGINAL: Mike Ledbetter
I used the Great Planes nylon mount on my US 60 and it lined up perfectly witht he factory holes.
His is a 40 size. Are you sure both mounts are the same?
If not do this: Get a Dave Brown glass mount sized for your engine. Put the two mounts back to back and mark the holes from the old mount's firewall holes to the new mount. Viola!! You don't have to screw around with the firewall and blind nuts. I got this idea from another RCU forumite after I already redrilled my firewall. DDOOHHHHH !!!