Would like to know the distance between the forward face of the large cross member (to the back with the large hole) forward to the rear surface of each of the 3 smaller cross members directly under the floor. I'm not sure mine are arranged the same. I hung my 25 gallon water tank from those on the other (driver's) side.
If I understand what you're asking, I think I have measurements that will help, taken from a 2014 2500 144 4 cyl, measuring the "cavity" in the photo you posted.
For clarity, assume the "zero" position is at the rearmost part of the cavity, just where the rear leaf spring mount and other stuff comes into play. So from this zero point:
+ 8" to edge of first cross member
+ 2" width of first cross member
+ 12.25" to edge of second cross member
+ 2" width of second cross member
+ 17.25" to edge of third cross member
+ 2" width of third cross member
+ 8" to where cavity ends at forward end of vehicle
= overall length is about 51.5"
Hope that helps. You may want to check the distance to the exhaust pipe/muffler, which varies a bit depending on where the battery may be place fore/aft on the chassis. The chassis beam that is flush with the bottom of the battery/tray in your drawing is within 2" of the exhaust in places, if my memory is accurate (it may not be!). Even so, it may not be close enough to matter.