More fun ensued with my good old Makita angle grinder and a wire wheel, followed up by some drill and air chisel games. In hindsight, I probably would have had the frame blasted and saved myself untold amounts of time, but I hear this is supposed to be part of the fun
I crosscut the heads of the rivets to make it easier, but still had to drill the middle out on some. Whoever GM had installing these things should be in the Hall Of Fame!