You would have to widen it slightly straiten the s curve and resend it. would easier to take the 4in I beam cut/ mill to the proper thickness and bend your self than trying to modify a pre bent one. That's how he made his. Find someone close with a m2 and transfer the shape to a hard surface ( heavy card board light plywood) then draw it up if you have someone with a water jet to cut the beam then it probably won't need to be finish milled.
The rail came in several pieces and was welded together
That's true for both early and late al stifle was making a set of scout car brackets don't know if they are still available ?
Halftrack has a few different ones. If you can weld then going with the early style might be your best bet.