Unfortunately, tearing apart the inner fender is the only way to do it.
Inside the body, the "captured" nut that the subframe bolt threads into will break free and spin.
The nut, which is square, not hexagonal, is held in place with a small piece of sheet metal welded over it to keep it from spinning.
On the assembly line, when all is new, and the sub bolts thread in, those captured nuts work just fine.
After 30 or 40 years, no dice getting them back out.
It might seem like a lot of work, and if you're lucky, it's just the fronts.
If the rear subframe bolts strip out, the only way to access the captured nuts is to remove the front seats, cut a hole in the floorboard above the nut, clamp the nut with a pair of ViseGrips, and then try to get the bolt out.
Not sure if any aftermarket suppliers have replacement nuts (that you would have to tack back in), but when I did mine, I held them in place while I chased the threads with a tap.
After that, you have to weld the little piece of floorboard back in place.
I know it's a lot of work, I had to do all 4 of my subframe bolts!
I replaced my mounts with urethane instead of stock rubber and installed subframe connectors at the same time.
