Hello, I have a 1985 gmc C1500 with a 350 and a 400 tranny. I would like to know what are the diffrence between 1500 and 2500? What would I have to swap in to make it a 2500. It originally came with a 4.3 v6. It also seems to have bed sagging on the drivers side, body/bed mounts?Any info good, thanks.