[watahyahknow]

looks like there was water in there , the rubber seals where probably damaged during the fire have water from dousing the fire pour through the cab floor and maibe having the truck parked out in the rain before you bought it without windows probably didnt help either . you can dunk some diesel / waste oil or so in it and cover the metal parts up then seal it up with some plastic and tape to keep the worst of the rust and junk out but you whont use it anyway and pulling the driveshaft out will cause the oil to leak out again making a mess on youre floor
the gearstick usually isnt atached to the cab just sticks through a hole and it covered up with a rubber covering the gearlever is usually thightened with two bolts on the top of the gearbox , could be the same dust cap rubber has glued the cab to the gearstick
the older trucks have prety thick sheetmetal , you might be able to save that cab with a good sheetmetal wordker and some cutting slots and rewelding to get the expanded metal and buckling ou