I am with Paul on this one. Having done an engine swaparoo before, into something that was never intended for that power plant was a HUGE PITA.
I am a big proponent of getting the right vehicle, then changing the easy stuff. Then again, if you are really into the looks of the older buses, you are either stuck with the existing piece, or you can gut it, and put in a 5.9, which are as common as dirt anymore. Just about every junkyard will have one, or more, it seems. Then you start by figuring out where to weld in some custom engine mounts, getting all new lines, the piping, the cooling, whew, it adds up.
Let us not forget the nervous system either, the WIRING. Oh man, how many beers have I killed with a pile of wires at my feet, by a wood fire. Beer in one hand, two manuals in the other, wires around my neck, and a labeler... somewhere in that mess.
Not going that way this time. Just getting what I want. Then, gravy.
So, in short, no easy swap on that beast. I wonder how far off the 396 is from say, a 6.5 liter chevy. Ugh, that engine is a dog too. Never mind. And getting harder to find parts for.