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I think the side walls of the original channel could be drilled and a new assembly consisting of plate steel and a new pre-welded foot could be used with good results. You have the opportunity to reach several inches up into good metal to do this. I would be careful to not unduly pinch the sidewalls of that original channel when tightening up the through-bolts. Yes indeed, prime and paint everything, and I would use stainless steel bolts just because they are strong and not a cost issue with this volume.
The actual loading on this assembly would most likely be far lower than the shear on those stainless steel bolts. Upon installation, I would also use a pair of wood wedges dipped in epoxy to make up the final difference between your new metal foot and the fiberglass ridge you are bearing on, and make a nice snug fit for bedding.