Crossmember rivet removal help

[Smokeem]

I’ve been trying to remove my trailing arm Crossmember to flip the mounts and weld pipe for exhaust to pass through. I’ve got all the bottom rivets removed and the front of the top 2 removed and the heads knocked off the back 2. I’ve been tried to drill the back 2 rivets butvthe bit keeps wanting to walk. I can’t get my grinder in to flatten the knocked rivet down. Anybody have any suggestions that might help me get these back 2 out?
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