Somewhere in the timeframe the diesel pick-ups were built (78)GM went from having a separate BOP bolt pattern to everything going to the Chev bolt pattern. It may be a non-issue. If he has a lot invested in the transmission, it may be worthwhile to use an adapter, but it may require the transmission mount to be moved back to compensate for the thickness of the adapter. It would probably be easier just to find a Chev trans and he can use all stock parts.
He will probably also need to get wiring schematics for both gas and diesel as the diesel has some modules, such as glow plugs etc that he will want to eliminate.
