I think that Ray had put copies of the Chassis dimensions on the site years ago. Did they go away with the new system?
The place where I worked scrapped a huge cast iron table that was used to inspect automotive stampings. You could sit an entire Ford van on it. I was in China following tooling when they took it out and sold it for scrap. I am sure one of the NASCAR shops would have loved had it. When I would go to the Ford assembly plants their quality room always has what is called a Buck for each vehicle. It is basically a huge assembly fixture. You do a set of panels at your shop and do what they call blue and scribe panels. You have a fixture at the stamping house and you put on CMM and scribe a 100 mm grid on the parts to the XYZ coordinates. Every part on the car is designed from the ZERO, ZERO, ZERO of the XYZ. So you go to the assembly plant and they put your parts into the buck which uses the same two way and four way locators and nets and clamps. So with the blueing on the parts and scribe lines you can see if your part aligns to the adjacent parts at a glance. Of course you have to supply the inspection reports also. After the quality and assembly people at Ford have viewed the drill holes and screw the parts together and you end up with an entire chassis as a screw build. This is where you will find any radius or trim edge that might interfere with another part. If you ever go into the BMW facility in Spartanburg / Greenville, S.C. there is a row of CMM since they do several different models in one plant. Same at Ford.
So after the screw build they will do some limited builds with first parts. As a supplier you have to be standing there as they assemble the vehicle. If anything does not fit or clear you are on the hot seat. I have seen errors in the Ford weld assembly fixture that our parts would not fit and of course you as the part supplier are always assumed to be wrong, lol. After a few quick measurements the Ford engineers saw their error. They take those bodies through weld then paint and assembly all the body parts. After the engineers review them they take out back and crush them. They do take some through final assembly and interior and drive trains go in for testing.
To build an accurate Chassis JIG at home would be quite an undertaking. Unless you have a laser scanner to check things with would be difficult to hold tolerances. With any mustang the first place it needs to go is to a frame shop and pull the chassis back to zero. They place I use can pull I think 45 tons in any direction. Some of the newer frame machines are quite complex. Most vehicles today cannot have the frames straightened. My son drove his new F-150 off about a 2' drop on the side of the road and the left front 20" wheel exploded causing it to blow all the side air bags thinking it was rolling over and it did not. When the Ford dealer put it on frame machine and checked it the frame was twisted and they would not pull it due to type of steel it was made from so totaled it. Same sort of thing happened to a friend with a Chevy dual wheel crew cab. Not a bent piece of sheet metal on the body but the Right Front of the frame was bent and they would not pull it. Totaled it would have to change the whole frame to fix.
The gauges you hang from the chassis are quite expensive to buy also. I buy most of my stuff from Auto Body Toolmart they carry a full line of the gauges you hang on the chassis. Go to autobodytoolmart.com to view.