I couldn't say about the 40's truck. But I'm guessing that it has a smaller cab, more rounded, etc. If it were just a "patch" in the center, should be easy. But would depend on how far it goes down toward the windshield, back window, and the door edges, etc. What I'm saying is, it is fairly stiff and difficult to fit into compound curves, corners, etc. My rear panel was the worst one. Curves side to side, back to front, and tapered front to back. That one took 2 days, several patterns, and 2 tries with the pvc sheet. I had a pattern from the Truck Shop for the front one which gave me a good head start. Didn't finish off the front edge, waiting to get the windshield glass in and the inside trim pieces.
Richard