ok , here it goes. i'm not an expert, but i will share what i have learned. the spring center to center on all models is 26 13/16". so the axle will fit between the springs. the 1 ton springs are 2" wide and the 1/2 ton springs are 1 3/4". so the mounting pad u-bolt holes and the shock plate on the axles will be different, and the 1 ton u-bolts are 9/16" dia. while the 1/2 ton are 1/2 " dia. you probably can't use the 1 3/4" springs without changing the spring end and shackle perches on the frame [different width].. also, the model 3100 and the model 3600 have a spring length of 38", while the 3700 through 5700 models spring length is 40". i hope this all helps. garyg
