If Ali began his career in 1948...

Ignore the social differences of the time periods, and the probable lack of funding that Ali would have received early. Assume he has comparable backing, same trainers, same style, etc. How would Ali have done if he'd turned professional in 1948? Where do you see his career going?

 

I can't see Ali reigning until the late 60's. In our timeline, he won the title in '64 and held it off and on until '78, after which old age and Parkinson's wore him down for Holmes. Assuming he reached the title after the same number of years as a professional (1952), he'd be in the same shape that he was against Leon Spinks by 1966.

He also got three years of R&R during his "exile" in our timeline. Here, he's fighting constantly from '48 onward against Walcott, Marciano, Patterson, Ingo, Liston, and Terrell in their primes. Plus, he'd still get Parkinson's.

There's no way he lasts until Frazier in '69.

 

Looking at the top contenders...

1952 -- Both Ali and Marciano won their title shots about four years after they started their respective pro careers. They'd collide somewhere around here. Very risky fight with a younger Ali; Rocky would remain near his prime until '56. They may not face each other until after Rocky beats Walcott. I'll give Rocky the benefit of the doubt and say that he beats Ali the first time.

1953/54-- Ali beats Marciano, and then beats him in the rematch. All three are grueling, classic fights. Ali keeps getting better during this period. Might fight Valdes or Walcott. The latter would be too old and worn at this point to make it interesting.

1954/55 -- Ali would beat Valdes. Maybe defend against Charles as well, although he wouldn't have the close fight against Charles that Marciano did to sell the bout. I don't foresee any problems here. If a Marciano rubber match didn't happen in '53/'54, it would happen now.

1955/1956 -- Ali would beat Moore with more trouble than he had in our timeline. D'Amato might keep Patterson away from Ali for another year or two. If he doesn't, Patterson would put up a surprisingly good showing.

1957/58/59 -- Ali would need to fend off successive challenges from Patterson, Folley, Ingo, and Machen. He'd be slowing down a bit by now, and all of these would be tough. I could actually see Patterson winning a close decision against Ali by the late '50's, since Ali would be at least as worn down as he was in 1973 or 1974 in our timeline. Patterson would have the youth, swarming style, speed, and left hook to make this interesting.

1960-- Ali might have enough in the tank to beat Liston the first time. Might fight Patterson again, depending on how their last fight had turned out. Don't know how that would turn out.

1961/62 -- By this point, Liston would have won the title. Ali would be a bad style matchup for him, but he'd either beat Ali directly, or beat Patterson after Patterson beat Ali. I just can't see an old Ali surviving the simultaneous pincers of a prime Patterson and a near-prime Liston.


His final victory tab includes:

Walcott (probably)
Marciano (either 2/3 or all 3 times)
Charles
Valdes
Moore
Patterson (at least one defeat as well)
Johansson
Machen
Folley
maybe Liston


...Not bad at all.

The 1950's/early 60's are remembered as a "golden era" for the Patterson / Ali / Marciano / Liston quadrangle. Nobody cares much about the late 60's to mid-70's.