I'm trying to catch up in this thread so here's some commentary:

My 1978 B300 has a 2245, R7871A. For the 1979 (I don't have the service manual for 79), I see listed R7590, R8597-R8598, R8925, R9816. Not the one you listed, they appeared to be 2245s. You definitely have the stud upside down there, possibly someone in the past replaced the carb with a different one which just happened to be available but not the correct one.

I am still leaning towards timing / distributor being your issue.

Someone suggested a soft failure of the fuel pump, and these can be difficult to assess visually but I would expect a rough/lean run in that instance but not a "no start". I am assuming of course you have checked the coil? Sorry if you may have mentioned it but the thread is getting long!

regarding 4 bbl swaps, when assessing a problem I try to change as few variables at a time during the diagnosis!

Keep us posted!