Bed to Cab gap question

[Conundrum]

I am hopefully going to be shortening my frame soon for the short bed conversion on my 74 GMC.

And I was wondering if I could get the amount of gap on some of your trucks between the front of the bed and the rear of the cab?

I would like to close up the gap some and figure if I can get a few measurements then maybe I could come up with an average that I could add to the 14" that comes out of the middle of the frame already. I figure on coming up with an average, because i'm sure that not all trucks are exactly the same.