[dfwright]

I think the key is that magic 30" line for the upper trim, and 10 or 15" line for the lower. According to the cab drawing, it looks like the lower body line starts at 5", so the "30 inch" line for the upper would be 25" above the lower body line, and the lower 10" line would come in at 5".

Gauging by the dimensions, the rear holes on the back of the cab have the following:
The upper hole is 1.09 below the 30" line, making it roughly 24" above the body line, and the lower looks like 3.3" above the 10" line, making it about 8" above the body line.

I went out and did a non-scientific measurement, and these seem to be close to what I have on the truck. I'll post a couple pics -- sorry for the lousy cell phone quality