Jersey, ah the rust belt!
I'm from Nova Scotia so I know your pain.
Unfortunately, I'll bet that bolt is not going to come out without snapping. especially on an 88. That's been in there on east coast winter roads for the better part of 25 years.
heck, you're lucky you even have a bolt left in there at all at this point......
Still, have you tried getting in the bed support and hitting the bolt from the top with the penetrating fluid?
Penetrating oil will pull into the joint through capillary action, but soaking it from the top is the best and then gravity helps pull it in.
I'm also willing to bet your captured nut in the bed support rail is in bad shape. Maybe even the bed support rail itself.
After letting it sit a day or two (keep hitting it with the oil every couple hours) and then try shocking the bolt by striking the head with a hammer.
Then you have two choices;
1. Breaker bar
2. Impact gun
Myself, I say impact gun and hope the captured nut doesn't spin. If it does, you have to find a way to cut the head off the bolt and then remove the bed. Once the bed is off, removing the bolt and nut is simple. Replace the nut with a new one and tack it with a mig.
The breaker bar is long and difficult. It also doesn't shock the bolt as you turn. FYI, you want it to be shocked. as in hammered. This makes it easier to turn through the rust. A breaker bar may do exactly that; break the bolt. In which case you're back to remove the bed and then replace the bolt and nut.
Or, remove the tank and replace the straps.
Or, find someone with smaller hands.
good luck.