[Dad's GMC]

Hi everyone. I have a question about Dad's GMC which was newly painted, brought home covered on a car dolly (not to scratch it up) and has been housed in a garage for the past 10 years. It gets started now and again (always rolls over 1st turn) but that's it. It has heavy duty front/rear springs (coil) 350 V8, 4 spd, 411. He modified it by adding air brakes. There is no sticker on door pillar. Husband has been buying tons of parts (tilt, chrome, carburator etc) and is hoping to doll it up some. Can find tons of boxes of parts and pieces he's ordered but can't find the darn paper work to tell me what size camper we can carry on it and tow the 1200 lb boat. Want to put the truck back on the road for summers but need GVWR before buying camper.
Anyone here got same/similar truck that might answer this question??

Thanks
Nell for Kevin