"At what point in time did Chevrolet begin using frame numbers instead of engine numbers? "
The 1956 and later required codes/numbers/IDs being pressed into the frame (at the factory). Maybe 1955-2nd and later.
If you want a link to a 1956 truck manual, I can try to find it on-line and I will try to post it.
Maybe someone else will kindly look for photo of that page in a manual on-line?
