Just had one of those "duh" moments going back to my dealership and factory days.
I see how so many of us custom build items and often there is a need for really good reference drawings with measurements
The OEM's put out what are called "body builders manuals/guides". They have dimensions and are loaded with top notch line drawings. Suspensions, frames, cabs, hoods, tanks, exhaust, blah, blah, blah. The are supplied to all the outfits that modify a truck once it leaves the factory
A rivet counters dream.
Here is a sample from the current Peterbilt body builders guide.
With a little digging, they are out there.
Good luck and hope it helps.