Although I'm on the boxing side, I almost think Tyson would be an "easier" matchup for Royce. Tyson relied VERY heavily on upper body movement to get inside and deliver the killer blows. I think this fight would be more of a coin toss than your traditional style boxer would be. It would basically depend on whether or not Tyson could drop Royce as Royce was going in. Maybe he would land that shot a majority of the time (and literally kill him with it, as you said). But it would basically always come down to that one moment when Royce tried to shoot on him, at least in my opinion.

But Ali would destroy Royce 9.5 times out of ten, in my opinion. He was the kind of boxer who uses footwork to avoid his opponents, and lure them into him, while destroying them with careful shots the whole time. Royce wouldnt be able to get his shot in, and hence would never stand a chance.

So like I said, Tyson would most likely win, and I would bet on him every time. But I wouldn't bet the house on him ever. Ali, maybe so, simply because grapplers tend to like the guys who come inside and work for the big punch to end it all. Ali wouldn't give him that.