Errrr...I've done a ridiculous amount of research on Bruce Lee over the years and there's a couple of things I just want to respond to.
1) Bruce Lee was ripped because of how he trained and how often he trained. He trained every day. He trained everything. He worked on cardio, weights, fighting, etc. He was always looking for new ways to push his body. While watching tv he'd be doing the splits, curling a dumbell in one hand and reading a book in the other. He was dedicated to the point of obsession. From what I've gathered through books, notes and the writings of his wife...his diet was nothing special. In fact, he could hardly cook so he just ate what his wife prepared. He DID take diuretics at times.
2) Anything can happen in a fight, but Bruce Lee was not just a movie star. He was definitely focused (once again to the point of obsession) with becoming one, but his interest in fighting was not a show. The man studied, adapted and evolved his fighting still his whole life (as short as it was.) You could definitely call him a "mixed" martial artist. He realized that he needed to learn more and employ more (including wrestling) to be the best fighter he could be.
3) Ali was a big, powerful and fast man. I'd hate to see Bruce box him, because we all know that would be murder. But a fight might be really ugly. Bruce didn't sit do flying jump kicks and flips when he fought. He went for the eyes, nuts and throat (documented). He was out to win, even if it meant crippling, blinding or killing an opponent. He trained for it. Baring a fluke, I think Ali wouldn't fare well against a guy like that in a fight. Throw in some rules and you may have a different story.