Towing Capacity of my truck?

[82GMC4X4]

Hey all,

I purchased my 82 K10 last year, and now I am seriously thinking of towing with it. From what I was told the original owner towed his 5th wheel camper with it (Holes in the box prove a 5th wheel hitch was in there)

The truck is a LWB 4x4. 305, 3spd trans, tranny cooler, 3.73 rear end and Big Ten package as well. How heavy of a camper could I pull with my beast? I was looking at 5th wheels as I like the room they have and they seem to have alot more room than a hitch type. Either way how much do you guys figure with both types of hitches. Thanks.