When you run your replacement hat channel, (the 1" square tubing) one thing that comes to my mind to consider. When you drill for the mounting hardware, over bore the hole, and sleeve with section of pipe. This would add to the overall strength of the frame. Do you have to, no. Your 1" square is plenty strong, but the pipe will prevent water intrusion into your frame work. When I replaced the hat channel on the M38A1, the original hat channel was strengthened with pieces of white oak. Being that the heeps were never designed to last forever, that was a cheap and easy way to reduce costs. The down side, water soaked into the wood, and dissolved the hat channel from the inside out. Long story short, I didn't use any non ferrous material when fabricating the channel. I did cut some steel pipe spacers to prevent the hardware from deforming the channel when tightening. The channel received several coats of primer and rust preventative before being welded into place. Another quick tip for when you fuse the floor sheet metal to the sub frame, use weld through primer. The stuff works really well, in my opinion.