A 12H-T was designed to go into a 60 series, and it was only built for a couple of years. Great engine IMO, but not enough of them (or parts) worldwide to justify swapping it into a U.S. vehicle.
An 80 needs a 1HD-T or 1HD-FT IMO. I've seen some threads on MUD of folks swapping in those Isuzu diesels into 80s. I think a couple of them have finished and are driving them.