On the frame, and if it has the original trans on a plate on the trans. I think the frame number is near the steering box.
My brother-n-law has a 42' ford p/u and we found the ID stamped on the LFT front of the frame down by the steering box. Try taking a SOS or a Brilow pad and rubbing down the frame in that area and you should find it.
I have a 41 ford p/u and the ID is stamped on the drivers side on the frame by the steering box. its stamped on not on a tag.