Forged cranks flex less at high rpm's and are easier on the main bearings, less wear, last longer.
The 273 engines were designed for going 3,000-4,000 rpms on the freeways and lasting 100,000 miles. They had solid lifters too, very mild cams but the was always upgrade cams available.
Then came the oil shock of 1973 and slower speed limits, so then cast cranks work just fine. Today, few people run 4.10 gears and no OD and drive 75 mph min on the expressways and if they do its not everyday for long distances