I carefully pried up the bolt's large square washer, using a wide blade drywall taping tool first. They are thin and flexible. Every single one popped up. The other route is to use a punch from the bottom and drive it up.
But I did not grind off any of the carriage bolt heads.
On all of them, the nut on the bottom snapped off. If the nut is still there, and keep turning it. You probably will have to use a punch on the top side to drive it out.
Three out of four of the clips on the frame broke off (small pieces of angle iron).
In the picture below, I show one of the bolts, two of the top side square washers, and two items that look like rotten washers, but are actually pieces of the angle iron tabs.
1977 Trillium Progress by
wrk101, on Flickr