I was a GM guy for years . Owned many . I knew all the tricks , swaps , change overs . After the bail outs I sold all my GM's except my 53 Pontiac ,My first car , and bought Ford . Sorry for the long intro . I now own a 78 F100 that a friend left me when he passed . I rebuilt the engine, some electrical work , and drive it almost daily . The rear springs are shot ! Any load at all and she squats . The rear sits lower than the front unloaded . What will interchange ? 2 wheel drive , long bed , 302 , c6 . I would like to beef it up a bit for hauling firewood and home improvement items .I don't want to make a heavy hauler , just a bit better than stock .

 

I would try springs out of a f150 with the tow package. Maybe replace the shocks as well. I believe the f150's had a little bit heaver leaf in the back.