Any body replace front frame rails on a 71-75 A body

Thanks for all the info. I need all the help I. Can get. I will be taking the core support off, but welding it back on to the new rails. The aprons I am undecided about. The reasion I am having to replace the front frame rails, is because some time ago. Some body installed a mustang II frontend and to me didn't do a very good job at it.
I say that because I was measureing the frame width to see if a motor plate would work with my magnum 360. That's when I found out the mustang frame rails they made are about 4 inches narrower than the factory frame rails. Pluse they didn't install the cross brace. I was thinking about just cutting the frame rails and putting them wear they need to be, but said to my self if I am going to do that might as well just put the factory rails back in it?