Spinning Bolt

I'm restoring a '72 convertible, and as I was taking out the interior today, I couldn't take out the drivers seat. The 2 rear bolts holding the seat bracket down both just spin in place. I tried gripping it with some vise-grips and pull it out, but it wont give, and I cant wedge a screwdriver under the bolt head. What should I do?  

well... a few questions...

is the nut all the way through the floor pan???? where somebody has to be under the car holding a wrench to loosen the bolt from the top.????

take a look .. this has happened...

what are your plans for the carpet????    going to save it???? or is it being replaced..

more than likely.. if you can run the seat far enough forward...    center punch the center of the bolt head...    then create a large pattern of low tack masking tape strips in a tick tack toe pattern to catch any drilling fragments..    drill right through the center of the bolt head.. you don't have to drill very far.. you might start out at 1/8"... centered...  then come back with a 1/4 or even go 5/16..  as the 1/8 inch hole will act like a pilot .. and allow the drill to dig in deep quickly..   you might have to hold the hex with a pair of vice grips..    you don't have to drill all the way through...  and leave a stub of the threads sticking up...  

now that the seat and the carpet have been removed...

if the stub is still sticking up...     do you have a welder???    if its a captured nut in the floor pan..

drill 2.. 1/8 or 3/16 holes on either side of the old bolt.. just through the sheet metal...  you can now weld through these holes to reattach the captured nut to the floor pan..     depending on where you have drilled...   you should lay a plain 5/16 nut on the top of the stub.. and weld through the middle.. to attach it to the stub..    with a shot of wd40... after welding through the holes in the sheet metal.. you might be able to back the bolt out of the captured nut..