Opinions on a restoration

I would go with mar-tay's suggestion unless you have a professional do the work. Too much latitude to do a serious screw-up with either roof or frame rails and completely ruin the car (s).
Dragging a posse of 'volunteers' in to help out usually ends badly, no matter how well-intentioned.
Not meaning to burst your bubble, but them's the facts, Jack.
Economically, I would not buy another frame but instead put the money toward paying a pro to do the '74. Replacement titles have a bad way of not getting issued these days. Replace body parts to make it look like a '72 if that's your preference.