I started this process of mounting this today and here was my findings.
When I mount everything according to the drawing as shown in this picture, I will need to drill 2 new holes per side into the frames and bellcrank ribs. Rivet spacing should still be ok at .25 in o/c. Additionally, I will convert the middle 470 rivet to a 426 flush rivet to remove the conflict.
When I utililize the existing holes ...you guessed it the mount sits 1/2 inch proud of the bellcrank mount. The spacing for the bottom two holes results in 1/2 inch too low. I think this might explain why the mount from Garmin is 1/2 in taller...to mount in the bottom two holes.