I'd have to pick Lewis, though Byrd's boxing skill, quickness and mobility might pose some interesting problems for an aging Lennox.. Might be a tougher fight than people think.
Byrd wasn't a defensive genius like Jimmy Young,...yeah, he would have survived for a while, but like he was caught by Ike and Wlad, Lewis would have caught him too,...and unlike Young, when Byrd got caught, usually he went down.
I think a speedy fighter who moves well would be Lewis worst nightmare but Byrd did not fit that mold. He was flat-footed and countered in the pocket, too bad he fought guys that towered over him and were able to poke him from the outside all night, he never handled size well as his fights with McCline and a shot Golota show, as well as the Klitschko fights. Lewis destroys him.
Byrd would do well to make it to the final bell. His style just doesn't match up well to Lewis. He doesn't have the power to hurt Lewis, he doesn't have the size or jab to compete on the outside and inside he'd get mauled by the very physical Lewis. Lewis would keep him on the end of the jab and eventually Byrd will tire against the bigger, stronger man and get caught and then it's over.
Lewis would have a 10 inch reach advantage. Someone in an older thread said that Byrd would parry Lewis like he did against Wlad. How's Byrd gonna parry someone with such a big reach advantage?
Bryd had some good elusive technique. But the accuracy of Lewis would more than nullify that. Lewis would temper Byrds head movement by landing his jab, and setting up the distance for the power shots to land. I could see Lewis landing an uppercut in the 5th to end it.
Byrds defensive tactics worked on cruder giants like vitali but no way will they work on a skilled tech like Lewis, who will close the gap early and land to KO.
This is a pretty good example of the other thread " Why Does Size Matter " Byrd was far from a slouch, but the size, reach, weight and power advantages to Lewis would definitely tell.