What kind of header are you using,I use a headsnatcher 27 inch pans with a sunflower reel and stalk walker underneath header.Stalk walker keeps beating stalk down so all you take is head and 6-8 inches of stalk,3% DOCKAGE IS NORMAl.
Might want to be sure paddles on end of cylinder are all the way out and I would suggest to have some straight ones made up. You might want to get some paddles for discharge beater that are for the newer machines. I think you can find a way to fasten them to your beater. Fasten them with knotched side out and be sure they are at least 1 5_8" off of the floor. The newer beater and or older one with new style paddles will no doubt work better. You see the rolled back paddles you have now on both the cylinder and beater won't move the material very well without shouving it hard against cage or floor. You might want to see what you can do about doubling the belt and all that but I would address the reason it is pulling so heavy first. You must remember the next weaker link for belt is the next previous jackshaft which also got heavier on the 60 and 70. Also check the main double belt set on other side of machine to see the belts aren't bottomed in pulley just to inside of cylinder varible. There are lots of combines set up just like yours to do sunflowers with wet stalks and heads. Works very well but like you say you want to be careful about pushing to hard with concave blocked off. Good luck