The serial numbers are the ones that are most important. They tell you they are '42 heavy duty trucks. The rest or the numbers are just casting numbers, manufacturer stuff, most can't really be decoded or were known to the manufacturer. The 21T-5087 is a part number of that component of the frame. Ford never put door tags, so that tag was probably added at another time, maybe a fleet number or something, but not a Ford thing. Sounds like you have a couple of neat projects there.