On the 6622's in our area we put 8820 drive sheaves and belt on them and add a set of fan blades in the center, makes them a new combine and the sheaves and belt take an hour tops to do, I would start there as adding the blade is an off season job.
It the same sheave only larger, double check between the 7720 and the 8820, I always forget. When you get it all running I reset the stops so that the belt won't run on the spacers of the lower sheave. I can double check partn in a couple of days.
Ray Steuckle's book recommends "slotting" the fan blades on the Deere 00 series and sliding the blades out a bit so they are closer to the housing-thereby creating a larger fan and moving more air. To chicken to try it myself but supposed to make quite a difference.