Found this on another site... Take it for what it's worth...
Trying to remember if the #6 is the one with the V-belt in back or the flat belt in front. I had one with the flat belt,had a 1 row head,pulled it with an Oliver 77,about 35hp. Those were made for the JD As,about the same size as that Oliver. The gear box seemed to be a weak spot on those.The belt tended to want to slip alot too. The one with the V-belt looked to me to be a heavier machine. I've seen those being pulled by 4010s that must have been going 5mph in corn. I don't know that I'd try one in hay,they didn't chop fine enough to pack well for haylage. They used that type machine to chop dry hay back in the day though.