With a Mustang being of Uni-Body construction ensuring that the "frame" is square is even more critical than with a full ladder framed car such as a Chevelle. What most folks don't realize is when a unibody chassis takes a hit it will dissipate the energy through the entire structure. I have seen unibodies take a hit to the left front and welds in the right rear are popped/broken.
Any COMPETENT body shop with a Blackhawk or LaserLiner frame machine will be able to check it out. A fairly simple process-the vehicle is placed and blocked on the machine. Once a datum(centerline) is established then a framework with locating pins is lined up to key points on the chassis. Any deviation from these pins denotes a framework that is not square. I think the tolerance range is 2 or 3 millimeters.
Unless you have a solid, square baseline to build on then everything else will be a waste of time and money.
I was building up a 68 Nova until I built my shop and had to dedicate the room for the shop-sold the Nova to finance the shop. BIL has a 69 Fastback that will become an "Eleanor" clone, 68 Hi Country Special, 67 Fairlane ragtop and 68 Cougar with 58K original miles. The other BIL has a 69 Cougar ragtop.