It's been mentioned by other posters to just use the same setup. I agree. You have a stock 318 with AC and it's a cruiser. The engineers in 1971 deemed that cooling system adequate for that combination. They were pretty much correct. Not that there wasn't but I've never read any history of massive issues with cooling a stock 318/AC A-body in 1971. The traditional radiator shop has all but disappeared along with the old guys that knew what they were doing. Otherwise I'd suggest a good boiling or recore job. Just my humble opinion but a stock replacement radiator sounds like the easiest and safest way to go. Why try to reinvent the wheel especially on a stock cruiser.