53GMCPhil's comment about sliding the axle to the rear is a good one and it was always my first choice on a straight frame truck. Old chassis all had a taper over the axle that precluded doing this. Some of the 70's IHC & F*&d chassis had straight frames and it could be done easily on them but not on medium duty GM's. Actually it can be done on tapered frames too but it involves a lot of extra work that most customers simply would not pay for. Also I only did it on non dumping applications.
Any steel service company can form plate to the channel dimension you need so no reason to leave your locale. Just be sure the radius matches the frame so they must have the proper forming dies.
Your winch requirement is so small that it will not need additional reinforcement, Crawfish. I would NOT box the frame but rather do it as I first described by an angle inside the frame. You will need to add one additional crossmember to support the midship stub hanger bearing. Build it as close as you can to the original and it will not only work but look correct as well. Good Luck, I know you can do this.
