Byrd by a fairly long stretch. That said, he wouldn't enjoy this one too much; Old Foreman had a hard and nasty jab.
Both Foreman 1&2 would have his guts for garters. Byrd stood in the pocket . He had no legs. That worked against the worst crop of talent bums the division ever produced , it wouldn't work against Foreman. The uppercut gets him out of there.
He wouldn't need to keep Foreman off him, he would just need to outbox him, and keep moving. It is more the case that Foreman would need Byrd's chin to be bad enough to bail him out.
Byrd needs to be inside the Foreman wheelhouse to out box him. Is he going going to do that safely for 12 rounds? I doubt that. How does he get past the jab ? How does he deal with the massive physically imposing brute force on the inside. He gets herded into the ropes by the Foreman jab and lifted off the canvas with an uppercut. Foreman destroyed guys with quicker feed than Byrd. Byrds feet were too slow.
Foremans weakness was slick boxers. Byrd is not just going to stand there and let foreman hit him because of foreman rep of power. Foreman got a few gift decisions and was out boxed by Axle Shultz even. Byrd obviously doesn't hurt Foreman and wouldnt need a k.o because its not a knock out contest. I want to say foreman but I think Byrd would out point him, hand speed and fighting smart is the key here, Foreman one punch clubbing against a quick elusive guy, Foremans jab would find its mark but could he follow up with a Moorer one two punch and keep control off that one punch? I think byrd wins more than he loses here.