From memory (my parts book is out in the shop) there were a number of belly pans/skid plates for the 7U. Typically each was for a certain s/n range or arrangement (as they call 'em). Quite a few were home made as well so it's hard telling exactly what you have.
I'd use some large washers to bolt yours on. .I don't recall seeing anything about bushings in the book. Just to make certain, I'll check later this morning when I go out to my shop.
My parts books don't show any bushings. Bolts, washers, lock washers and nuts are the mounting hardware. I checked from 7U1 through 7U44307. I assume you have a Cat skid plate. Something else may take different mounting hardware. You'll appreciate the larger holes when you go to mount it. When those things take a hit and warp, it sometimes takes a bit of doing to line the holes up. Hope this helps.