Mike, You have a VERY difficult task ahead. I will try, together with guys like Kevin who have done this on AD, to help you. You say you have no welding skills. You will have a LOT of welding. To be able to weld, with most of it relatively thin material, you will need to practice a lot and even after that you will screw up sometimes on the actual cab. Burn thru and etc. You also have a truck that seems to be falling apart, hard to find places to brace and maintain geometry. So is it better to find a better cab? You could possibly build a way out of whack cab. None of us would find any of what you have easy to repair. So give it a thought.
The drawing is coordinate dimensioning. Using Datums. X Y Z (0's). The Y is an actual feature (firewall). Now-a-days we use part features for all three datums. But this is not a part. It's a weldment drawing. The factory would most likely build a jig to locate and clamp each piece in place for welding.
I would recommend considering using/establishing actual features. For X you can use the actual frame or 2 x 3 metal rectangle bolted to the cab mount locations to create a new X datum at dimension 10. I guess you would simulate the thickness of the rubber and washers of the cab mounting hardware by using hard spacers and bolt the two datum rails to cab. You have the 11.88 minus the 10 dim to double check the set up to floor. You then use the top of the datum rails, or the frame if used, and subtract the 10 dim from each other dim given on the drawing. This would also establish what would be the Z datum in the left hand view, because they give the centerline of body bolt to "0" (Z datum). You would have that body bolt.
This would build a sort of jig. Easy to build easy, to measure from. You could set the two rails on "saw horse" type supports to bring it to workable height. I just don't know if the cab, the sag you mention and the rust outs would even allow the rails to be installed. That leads back to a better cab. I just fear that you are a beginner taking on a journeymen's job.
This is no way meant to disrespect you or discourage your can-do spirit. I can not analyze the cab well enough to give sound advice. Just giving you help in both ways of thinking.
I will think this thru better and make sure I said it right if you decide to build a datum structure.
Maybe using the drawing is not the way to go. Maybe bracing, using dims from other trucks and measuring your truck is the way to go. Maybe the drawing would just be reference. Again depends on the ability to brace and measure before starting on a sagging rusted truck. You keep running into more and more problems, but so did Kevin and others, and they pressed on with success.
