If you drilled out the rivets, did you grind them down just about flush with the bottom plate. Get yourself a punch and a decent sized ballpein hammer. Tap the punch into the rivet holes that you drilled and ground down. There is probably a little bit of rivet surface holding the sideplate and bottom plate together on each of the rivets.
If you've already done that, then I'd look real close and careful for weld marks. While it's not a standard thing, I've heard of them being welded in the field as "Field Expedient" repair.
If that doesn't seem to be the case, then hopefully some others with experience taking them off the bottom plates will sound off with some other help.