Stigg,
You'll find it tough to get someone to cough up a plan for a jig like this. Basically you're taking the hardpoints of the suspension and running gear and creating a fixture to relate them in space. Using the Mustang as an example, the person with the jig figured out where he wanted the control arms located, the steering box and idler arms or rack, the motor and trans mounts, and any other critical features. Knowing the coordinates for where these were all located enabled him to build a framework (jig) to hold things in place while chassis tubes were welded up to connect them.
Where to get that info is a different story... Search around here and you'll find people asking for blueprints, with which you can figure it out. You can also buy someone's frame, create fixturing to assemble into it, disassemble the fixturing, remove the "pattern" frame, reassemble the fixturing, and then recreate the frame with pickup points held by the jig and fresh tubes. This might result in some unhappy people (IP infringements, etc), but I know it does happen (seen it done with circle track dirt racers where it's pretty common to wad up a chassis in a tangle).
I tore through an old Mustang in high school and in the back of the Ford shop manual, there was a fairly extensive list of reference dimensions for the chassis. Using these pages it is conceiveable to create a jig but odds are it'd be challenging.
The jig holds the mounting points in place while you weld them to a frame. Where the jig locates them is up to the builder to decide and based on intent and purpose of the project.
Not much of an answer but hope that helps somehow.
