ALL of the 350's I have torn apart that have came out of trucks (approximately 8)... whether 1/2 ton or 3/4 ton have ALL been 4-bolt mains. ALL were cast iron cranks. No steel or forged cranks. Automatics or standard transmissions, 2WD or 4WD, didn't matter. I always figured trucks mainly got 4-bolts to account for the thrust applied though the crankshaft from the torque sent to the input shaft of the transmission from pulling or hauling heavy loads. But I might be wrong on that.
Gary