I ground a bunch of those rivets last fall laying under The Yeller Dodge. I used some really aggressive carbides I dug out of the dumpster at work. I got absolutely covered with grindings. At least you have a bare frame!
yes the frames up on my trailer and that part is actually sticking off the back of the trailer so it will be a walk up to it situation. I'll probably start with my cut off wheel and see if I can't take a slice sideways at it...