Good info posted so far.

It's also a darn good time to replace unloading auger belt and platform reel pump drive belt and look carefully at the condition of all the other belts that you are removing to replace the separator drive belts. Also make sure you get your belt traps adjusted properly to avoid burning and scuffing the new separator drive belts.

It helps to have the special heavy tool for removing the hydraulic pump but is not necessary. I always use anti-sieze on the threads of the pump when I put it back on too.

I think I remember you said you did not have the correct manual for your machine. I'm always willing to help out a fellow 20 series machine owner with any details or help I can provide.

Here are the pages out of my 6620 TII  book describing the process. Your 7720 should be the same.  

Sorry for the huge pics but wanted you to be able to clearly see the dimensions and details.