I've run into a situation like that before and I ended up using a small hollow spacer slipped over the stud of the rivet. If I'm not mistaken the stud is designed to break off down inside the rivet body so spacing it up doesn't matter. I've found various spacers at Home Depot and Lowe's in their hardware section. You could even try #10 washers stacked up high enough to provide clearance. Just be aware that once the stud snaps the washers are going to go flying!
Basically anything that will fit in the area and still keep the rivet gun out of the way should work.
Air forums # 1674
1974 20' Argosy Motor Home
1974 31' Excella trailer (parting out, as of 4/1/2015 I have wheels, brake drums, windows & holding tanks left to sell)