Bought a truck need to stiffen up the rear

[LeatherMan69]

Ok so i bought that 79 GMC 1/2 ton 4x4 with 50K original miles on it. The thing is my dad wants to pull a ditch which trailer and a 3100lb tractor on it. The trailers tongue weight probably doesn't exceed 200lbs but with the tractor it adds alot of weight. My question is... A:will this trucks rear dip alot with all that weight, and would it be possible to just throw some stiffer 3/4 ton springs on it? Are they interchangeable? where could i find some cheapies? Any other ideas?

I also gotta weld a receiver hitch on it. Its got a Pental(sp) hitch on it. Not sure thats even the right name for it. But im thinking of taking off the old farm bumper and setting the hitch higher to help with the drop issue. Any thoughts?