In the early 60s the big 3 had 2-4s offered on their ‘top’ engines, Fords, Chevys, Buick, Chrysler. Trying to sell their lower engines in everyday cars and trucks. I imagine the 6-pacs (440 and 340) were made because the Hemi was expensive and not to durable at that time so Chrysler made something affordable and durable, kinda after the fact. All in my day and time and me so poor I couldn’t even pay attention lol