Frame rail options

I am in the same boat. I just bought a car that some body installed a mustang II front end on the 75 dart sport. My problem is should I try and fix what they put under it or put the factory rails back in it. But after reading this post, sounds like it might be harder that I thought. Meaning trying to find a good set of rails to weld back in it.
My problem is who ever installed the mustang II put it in 4 inches narrower than the factory frame rails. So I don't see me being able to use a shelf header for my magnum 360. Also I need to fab a better steering rack mount

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