Being an avid boxing fan and an avid RJJ fan I can tell you without a doubt that AS would KO RJJ in a boxing match. Roy does not have the speed or chin that he had in his prime. This match would be a a freakshow match in my opinion and just shows that AS does not want a real challenge.
Now if you match RJJ in his prime (approx 2000) against AS in his prime it would be a complete washout in favor of RJJ. Unfortunately his Heavyweight experience and age caught up to him and he immediately lost his biggest advantage, his speed and twitch reactions.
I love AS, but are you telling me Anderson's boxing is that good he can use it to defeat one of the greatest boxers of all time?