I got a 383 stroker, with a SM 465. It keeps over heating. I have been through two other used 350 and rebuilt this 383 it still runs hot. Next step is to overhaul a SM 420 I got replace the head gaskets and resurface the heads. I can get it done for about $400. Not more then 3,000 miles a year.
The engine was just fully rebuilt, all that was done during the rebuild process. No exhaust in the radiator I’ve already checked that. Re working the heads would just be a precautionary, so there’s no guessing. Transmission is old and worn out that may be putting extra stress on the engine. Diesel engines run at lower RPM’s and tend to generate less heat then gas powerd engines. A Cummins is hard to beat, and from what I understand the easiest swap would be to go with a BT. The 350 I had just couldn’t handle a 10,000 lb trailer load, so I rebuilt the 383.
One thing about the diesel, you will need a different transmission than the SM420. Those are strong but that unit has no overdrive gear. Since this is 2wd you could use a Dodge NV4500 or maybe better would be an NV5600.
Don’t waste your time with the 4bt. You can get a 6bt for the same price. You will need a diesel tranny in either case which doubles the price of your swap. You will end up spending a considerable amount of money for this swap but will not be disappointed with hauling performance. Though you will never “make you money back” driving 3k a year. My $.02