In CA they smog the vehicle as per the Frame. I don't think their is a law for what body to use, as long is it meets the general rules such as tire size, height of headlights, bumpers height, etc.
I don't know about the length of the Datsun, but if you're going for retro, why not just buy an older Toyota 2WD truck and build it up into a 4WD? You know how cool that would be-
Like this '75 Pickup:
Or even better, an old Toyota Stout, like this '66:
...........On second thought maybe not
