Hey I've found out recently that the (1/2 ton) rear ends on these trucks differed over the 63-66 model year range in the location of the panhard / trac bar mounting locations, both on the axle housing and the frame. I've never seen this listed among the lists of changes for these years, so you may want to take note. So does anyone know or care to venture a guess as to why they might have changed this up? Performance? Ease of manufacture? I've seen at least one picture of a 66 panhard bar and judging by the weird angle it mounts at, I'd guess the latter.