"Could a "grey market" truck be legal..."
Well, sure, given unlimited time and money! The major hurdles are emissions controls, crash safety items (airbags, bumpers seat belts), and other safety equipment (lights, tire rims, and so on).
Just getting your truck past USA customs is expensive. You'll have to post an large bond guaranteeing that the vehicle will be brought up to USA specs. If you don't, the feds will confiscate and destroy the vehicle.
It would probably be easier to adapt a USA-legal Fuso FE crew cab body to a USA-legal Fuso FG chassis, even if it involves lots of fabrication of new body mounts and so on.