In the course of removing a buch of rivets to move the transmission crossmember 2" forward, which made a whole bunch of stuff easier for my Cadillac into C60 conversion (and a couple things more challenging), I found the quickest way to remove them involves ear plugs, safety goggles, and cross cutting each rivet head so it looks like a Phillips screw. Then those four little quadrants knock right off with the air chisel, and from there it's an easy punch with the proper air hammer attachment and they come right out. If they won't come out you can drill them to release some tension, but I was able to punch most of mine out. It's super noisy so wear ear plugs
Also modern day titanium drill bits (the anodized yellow color looking ones) really make short work of old steel when you need to drill holes for the new bolts!
