Scott hit the nail on the head about the transition from box to channel. I've seen a lot of cracked frames right where a boxing plate ended that was square to the frame. The early for sure and maybe all S10's transition from box to channel somewhere near the back of the cab and the inside frame face, which is the same as a boxing plate, has a parabolic shaped end to gradually go from stiff to flexible. I haven't seen the 51 GMC with the 500 lately but do recall that the transmission crossmember is also a "K" member. It never hauls anything heavier than blueprints and never pulls a trailer so it probably isn't a good model for you.

Evan