1972 Ali tells 1966 Ali at the staredown that he's been diagnosed with a rare tropical disease that will kill him by 1973, that he spent 1967 through 1971 in jail, that his family disowned him, and that he's flat broke. 1966 Ali spends the entire time worrying about his future, and gets narrowly outpointed.
A solid 15 round UD for 60s MA; however, 72 Ali would give the younger man fits by dint of advanced ring IQ and better punching power. The younger Ali's foot and hand speed would spell the difference, far more accurate and voluminous punches thrown, and the older Ali would find himself having to chase the younger way too much.
The real question is, is 72 Ali fast enough to land regularly on 66 Ali. I feel the older version is slower but also much tougher and with a heavier punch. If 72 can land enough on 66 to make it count he may actually do him like dinner come the later rounds. That question aside, I'd put my money on 66 on points. I think he's quick enough to avoid the worst and generally the younger fighter with the same style is going to come off better.
In the 70s, Ali answered the question on television. He said the younger more active, better moving and more stamina version would have outboxed the matured wise version of himself.