i would go with as early as possible (ESPECIALLY if you plan on building a 408). the 360 was introduced in 1971 and up until about 73/74, the blocks were very strong and thick. They started using penny saving and weight saving techniques that really compromised the integrity when turning this 35+ year old iron into a high compression long stroke motor. Just my opinion, i have a 71' 360 block thats going to be a 408...someday! haha