I had the same problem with the rear one on the drivers side. I took my car to my favorite body shop and they drilled a hole through the subframe right beside the bolt ( and thus the nut too ) and then they spot welded the nut in place. The bolt turned right out afterwards. Put a new bolt in after you do this cause the old one will be rusty.
Cheers, Neil


96LXV6 heavily un-modded due to family addition. 87Renault GTA convertible, chopped springs, Ford Premium Sound, but otherwise stock.