The front of the rolls are shimmed. I believe the measurement is 4 1/4" center to center of the the two stalk rolls. To shim the rolls you need to remove the three 5/16" bolts that hold the shield in front of the rolls. Once this is off there are two 1/2" bolts that hold the front bearing support. You may find shims behind them. I have used flat washers instead of shims in a pinch. I have found that shimming the rolls does not make much difference in the performance of the rolls. Check the following.
Before you go to the trouble of shimming the rolls check that the deck plates (stripper plates) are centered over the rolls. Plates that are not centered over the rolls will definitely cause plugging. Also make sure they are open far enough so the stalks can go down.
If the stalks are lodging at the back of the deck plates and the deck plates are adjusted properly then re-time the gathering chains so the lugs of the two chains are directly opposite of each other. This will pretty well eliminate the plugging at the back of the deck plates.