Go 1500 springs. In my 81 i bought new diesel springs for that year[ 5.7, 6.2, 6.5] whatever they were, and they were Way to heavy. Dropping the engine in place dropped the front end only 3/4", i suppose GM's diesel that year was sugg. heavier than the cummins?? I even tryed 350 springs and didnt like the rate, ended up with new 6 cly. springs and cut 1 1/2 wraps off the bottom with a cut off wheel, and installed 3" drop spindles. This was as low as i could get the fron and still just keep the coil 'tight' in the bucket when the trucks front end is off the ground.