here are the issues I see with this:
I have two trucks, same basic truck, one has a SBC the other the 4cyl isuzu, the isuzu engine sticks out farther behind the cabover cab, and looks a good bit taller. that could cause major fitment issues in a cutvan engine bay. V8s tend to be wider and lower than straight engines, though ford was putting straight sixes into van engine bays well into the late 90s I think, just measure twice before you start cutting.
the cost of engine swaps is rarely the actual driveline, it's all the collateral damage. re-routing things, making stuff fit, fabricating linkages, etc. If you've swapped drivelines before, you probably know that, if not, this doesn't sound like the one to start on. Lots of odd things that may or may not work right out of the box could kill you. Fuel pump? fuel tank? Brake booster? ABS?
getting it inspected is going to be a nightmare unless you know someone. And then you'll still have to worry about some crafty trooper noticing the swap. In the state of MA you're required to have the truck inspected by a trooper for re-titling after an engine swap that isn't just an "insert part #XXXXX here sort of swap. Determining and PROVING the swap is safe where it's going to be hard. Any DOT people are going to want proof.
I doubt you'll get 20mpg, We get 12 or so with our small isuzu cabover.
There might be unanticipated wiring and electrical issues. Modern computer controlled cars can be a bit crazy. There are lots of electrical gizmos all over the truck that might need the ecm plugged in... lots of subsystems. Many are going to be required by law to work as they where orig equipment. AC, ABS, etc. They may sound simple, but if they're computer controlled at all, it could be a battle. Newer powerplants into older vehicles is easier than older powerplants into newer vehicles due to the computer stuff.
But... if you pull it off it'll be an epic swap, no doubt about it. Make sure you take lots of pictures of the process, it'll be cool.