Well I got yer answer, but it's not a good one.
That stud on rear of the seat is actually a bolt head. There are pos factory bolts that rust and desinigrate easily. What I ended up doing on my '84 was drilling that bolt out. Your seat rails are seized up I take it. Try some kind of rust breaker and a little muscle. I think there are bolts/screws on the bottom side of the seat that attach the rails but I can't remember w/o lookin. You might be able to squeeze a socket driver in there.
But from the look of things you will have grind the bolts off anyway, so I suggest bottom side of the truck and use a grinder(can't remember how much room they give you) and grind the left over bolt stud flat to the sheet metal of the cab and finish drilling the rest of the bolt out through the bottom.
I would try soaking those rails (If u want the seat that is). They like to jam up, but get a grip on it try using two people to yank on that bastard. If it comes loose just do your grind and drill from the top side.