I will stab at your 57 weight at about 3700 pounds with about 40 % of that riding on the rear wheels. In that case, you would be looking at about 1,400 pounds being supported by the rear axle or 700 pounds on each side. If you used 350 pound springs on your coil overs, they would depress 2 inches with an empty truck. The 150# tongue weight would only depress it slightly over a half inch which would not be a factor. Air bags would be ideal for what you are looking for and they work great on a Triangulated 4 bar set up. Recommend you look at and talk with the guys at Air Ride Technology for some good detailed info on what you are trying to do. They sell both coil over and air bags and should steer you toward what will work best.