I've been defeated by the front seat rear bolts in my xj. I can not remove it despite my efforts with a hammer chisel, heat, grinder, lots of pounding etc. I have access to it from the top and the bottom because I am currently replacing my floors. I haven't tried a drill yet because I am afraid it will just burn through bits.

Any suggestions?

 

Use a cut-off wheel and cut off the bolt from the bottom. Use a center punch and punch the old bolt out. Get a 5/16X2 bolt(grade 8 preferred) from the hardware store along with 2 big fender washers(one for the bottom, the other for the top).