Make no mistake, this ain't gonna be an easy project. I had a 71 Mach 1 that I was gonna restore and I quickly found that the 71-73 cars seem to be the "bastard stepchild" of the classic Stang family. Parts are out there, but you gotta look REAL hard to find them, and you really should save every link that you find as a success. They all come in handy later down the line.
As for a custom frame fab, that's uncharted territory for a LOT of people. I can see a lot of benefits in it, though obviously you wanna save anything that you replace so you can use it as a template and/or reference.
Good luck with what looks to become a beast of a build! post pics, dude!