Just to update you guys in case anyone is looking I found the number! It was on the top of the frame rail as we thought, almost right behind the steering shaft, near where the frame curls and bends down towards the steering gear.

It matched the numbers on the registration that followed the truck which makes sense. So the cab was definitely swapped at some point.

I will be beginning the titling process in the next few days. I appreciate everyone's help, I plan to start a new thread about the truck in general so I can share some pictures and ask some more questions!