i had to deal with this b4 and what i did was,remove the inner fender liners ,using a cut-off wheel cut a little access in the side of the framerail that i could fold back and remove the cage nut,remove broken bolt,reinstall cagenut ,fold back down ,reweld and seal,and paint,worked for me...good luck bro 
EO 98 CSVT, black, gunmetal exel 18s, H&R springs ,custom grilles, 35% tint,Hightower headlights,resonator removed,Magnaflow installed,FMS 9mm wires,K&N,Redline MTL
2000 Impala silver 2 inch drop 17 inch Exels
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