Help - Fuel sender mount bolts spinning

[grhanka]

The fuel sender is held down by four square-head screws caged in a metal bracket. The bracket is welded on, and is not tight enough to keep the screws from spinning. 3 of the nuts are frozen. I tried cutting slots for a screwdriver, and jamming a probe under to lock up the screw head, but I can't get a tigth enough grip to break the nuts loose, even with a impact driver. Any ideas before I cut the nuts off and have to rework the bracket.