Too bad GMC didn't Tocco harden the 478 & 637 crankshafts. International Harvester Tocco hardened crankshafts in the old Farmall H & M model farm tractors. Some just months away from 80 ( eighty ) years old are still running regularly with the original crankshaft. I've only ever seen ONE broken M crankshaft and heard of one other. Undersize main & rod bearings are NOT common in them.
I know GMC Tocco hardened 401 and 478 crankshafts back in the 60's, I have a factory shop manual that mentions it. In the 70's Tocco process was replaced with Tuftriding according to my factory parts manuals, they recommended after turning the cranks to have them Tuftrided, the posibility Diesel versions didn't have this process. When I had the crank turned on my 478 I told the machinist to have it tuftrided he told me that process was replace with Nitriding so that's what was done on my 478 crank
