OK I'll do it, I would guess somewhere around $20 000 with that box. A great base to build on, but without more details about mechanical condition and extras its real hard to say, I could be off $10000 either way.
A only-slightly informed guess from me as well, but I'd be a little stronger than $20K. A good 416 by itself could easily be worth that, and the box, if it is big enough for your use, is well-mated to the truck. Points off for rear- instead of side- entry, and for what look to be the cheapest possible windows. Points back for a cool ladder and roof rack setup that would make a good mounting for a roof-top tent. Big demerits if there are any leaks in the cabin or if the mounting system is dodgy.
So, $25-30K if the underlying 416 is good to go, though much less if it has any issues at all. And run away if the truck has significant rust.
FWIW, if you have not driven one or are not familiar with their driving and performance characteristics, know that 416s often aren't a good mix with North American roads and, especially with no turbo, North American terrain. My 416 camper, which had 110 hp and redlined at 54 mph, was generally not happy when subjected to either either Portland urban traffic or Oregon's frequent steep grades.