My frame vin is on the outside of my passenger side rail. If I remember correctly, its right below the door. I had a hard time making it out till I wirebrushed it, then I was able to see it fairly clearly.
My frame is from a 79 GMC 3/4 ton 4x4, with my old 2wd 1980 cab sitting on top of it. I talked to the DMV about that, and they said it wasn't a problem so long as I had a bill of sale for both vehicles, which I do. No titles up here

The 80's frame was too rusty, the VIN musta flaked off, cause I never did find it.