I think portions of the Chevy compatability actually go even further than that. As long as it's the basic small block and the engine mounts haven't changed, you could easily downgrade a modern fuel-injected engine to a carbed one without too much hassle.

In any of the apocalyptic scenarios, cover and concealment come to mind. Armor's great and all, but if you can't stay concealed you're going to have problems.

The diesel DUKW would probably work well for a good portion of this, but again, it's parts that are a biotch.

Maybe crafting your own floater would be the best idea?