What is the towing capacity for a 1979 GMC SIERRA C15 stepside 1/2 ton pickup truck?

I don't know how your truck is equipped but you should be able to tow that load with a V8. You need trailer brakes and maybe a weight distributing hitch.

There will be trailer brakes, a brake controller, sway bar, receiver hitch, and a weight distribution added to my truck. My truck has a 350 in it with a 350 turbo transmission. Both have been hopped up a bit. The transmission has a shift kit in it as well. I dont know if that will make much of a difference. I just dont want to tear up my truck or burn up the transmission by pulling that much weight.

I think you are OK. If you are concerned about the transmission, install a good aftermarket ATF cooler, deep transmission pan and a transmission temperature gauge. Make sure the engine cooling is working well.

I think I will be fine. The previous owner said I should be as well. He used to haul heavy loads with it when he owned it. He used to haul a car trailer and a truck which together weighed about 4500 lbs. He also didn't have all the extra stuff that I will have on it to pull it. He just used the factor towing. So with what I'll have with the extra weight and the weight distributor I should be fine. They are even letting me come tomorrow and and hooking it up to my truck with everything on it let me see if I can pull it and let me get comfortable until I decide to by it.