Very nice work. IMHO a very good example of properly done variant spam.
There are a few minor things that could be fixed, I'll send you the details but I have to ask something to Caswal/Andrew first about cargo subtracts.
Although I have some comments to make about the box of the pickup and I'll post this publicly as advice for other people.
First thing I have to say, the way you made it looks very nice by itself, the problem is when you take it and use it in-game.
The first issue is with the thickness of the walls, which are very thin in your case.
This causes issues when fixtures are installed, as they will tend to clip through the box. While you don't have to make them thicker everywhere, and they can still be quite thin at the top w/o too many issues, like on the not-El Camino, I would recommend moving the inside walls a bit further inside the box, and verifying that there are not too many problems with clipping.
In your case due to the very pronounced curve of the outside walls, you should be able to make something very nice w/o too many problems and w/o making the walls look super beefy like on this thing I am working on.
The other problem is with the platform clipping through the box. Due to how the game works, you have to adjust the geometry at the bottom to reduce clipping as much as possible. Personally I just adjust both the arches and the very bottom of the box until I don't get too much clipping. IIRC, in the case of the 80s not-El Camino, the only setup that clips through is multi-link rear suspension.