The Ali of 1966 and 1967 would have been a tough ask for any heavyweight in history. I think he would have been a bit too quick and light on his feet for the '74 version. '67 Ali wins on points because he's not knocking himself out (as weird as that sounds).
In all fairness to 'Mr 1974 Ali' - It was 1976 when Muhammad fought Jimmy Young. He'd slowed up a lot since he'd whupped George Foreman and was in terrible shape. 1967 beats 1974 on points. Too fast and busy. 1974 incarnation was still very fast for a heavy,though.
I think he already answered this himself in Hauser's book - I'll have to check, but believe he said he was stronger and had more belief in himself as an older fighter, but the younger Ali was too quick, could dance and move for 15 rounds and would score more points.
As an aside: how gracious was he to the British fighters? Nice - for all his braggadocio, he could be an exceptional sportsman when he chose to be ...
67 Ali beats 74 Ali But I would pick (pretty sure I will be a minority) 71-73 Ali over 67 Ali. Not because he was physically better, I just think he was stronger and had better ring generalship and that although he had a slight physical decline, his understanding and adaptability offset that.
74 Ali was far,far smarter and more of a seasoned pro than the 67 version. Of course you are never better 7 years later but styles make fights.