Pick-up trucks used a 20" inch diameter fan because they have a LOT more under hood room for a bigger radiator. I used a pick-up truck radiator in my '85 Impala by dropping the bottom radiator mount down two inches to get the hood to close. It allowed me a bigger radiator to cool a bigger motor.
With even a high performance (high horse) SBC you should have no difficulty getting the stock radiator width and height to cool your engine; just adding additional depth in terms of added rows of radiator core to handle the added heat load (stock two row copper/brass radiator for a base SBC; and a three row copper/brass radiator for a high performance engine or a stock one with A/C. If you need to go to a four core copper/brass radiator (used with a BBC and A/C) it is an option.