It IS something you need.
If your car got hit hard enough that you need to adjust the fender mounting holes, ie, the inner fenders, it got hit hard enough that you need to put it up on stands, level it, drop the plumb lines and check the frame rails. Because if it got hit hard enough to move the inner fenders, it got hit hard enough to move the rails. If the rails are straight, awesome, you have a solid foundation to work with. But you can't have the fender mounting holes out of whack without having other stuff out of whack, the radiator support, the cowl, the frame rails, something. And you can't properly square the radiator support if you're not squaring it to straight frame rails.
I'm sorry, but, if it got hit hard enough to worry about the fender mounts, you have to start from the beginning. Because that's what those diagrams are for. You have to start from the rails. If the rails are straight, you move to the radiator support. But you have to check that against the rails, so, you have to check the rails first. When the radiator support is square, you can move on to the inner fenders, because you have to check those by referencing the radiator support. If you use the measurements in the factory service manual, you can work step by step, referencing and projecting your way through the entire chassis and body.
And, like I said, if you pulled those measurements off the car sitting on a couple of rim stands sitting in the dirt in a wrecking yard, they're pretty much meaningless. Because in stock form you can change all the body gaps on these cars just by jacking the car up in the front. In order for those measurements to be useful they'd need to be taken on a car that was leveled and square, and that car sitting in the yard is definitely not leveled, and we have no idea if it's square or not. And that doesn't even consider that cars on this website, original cars with no damage, are frequently found to be as much as a half inch different on body locations than other original, undamaged cars. It comes up all the time fitting wheels and tires, the quarters can differ as much as a quarter inch from side to side on the same car, and I've seen 1/2" differences from one car to another. The factory body tolerances were not that great, they only worked from the chassis measurements they listed in the manual.
