[njsteve]

There are certain parts of an automobile that "The Law" considers sacred. Anything with a factory attached or stamped identifcation number is in that category. Lets use a Mopar as an example since they are unibodies but don't use a removable front subframe. Theoretically you could set up a jig in your garage and bolt your original rusted shell to it. You then cut away all the rusted parts from the firewall back and weld up the back portion of this Dynacorn body to that firewall. Then once that is done you cut off all the rusted stuff to the front of that firewall and attach the front half of a Dynacorn body. Technically you would be within the law (and out a lot of $$$ in labor)
The moral is because of the illegal acts of a bunch of people over the years, we are all stuck with these strict guidelines.