How many hours on the engine, I would check all my options before taking her apart. You may have an external problem that is causing the vibration. The book you need probably could be copied from the dealer.
Before tearing into the motor check the drive plate to see if it is the problem. Next check the compression. I would not mess with an after market book, get the right shop manual from AGCO. When you are working with a motor you need the right info.
6070-6080 AC tractor eng was the same as to specs. A vibration is really abnormal. Check the flywheel as suggested and the front pulley. The only other item is the internal balancer. It drives off the center gear pressed on the crankshaft. A black eng has bosh injectors and is pretty clean running. Check spec's per service manual as to injectors and timing. There was a service bulletin on the later higher compression pistons. The late black engines also have the newer brgs or bushings in the balancer. The op man has detailed instructions as to setting the balancer and its specs whic are also important. let us know what you find. (check the engine mounting bolts front and back as well for a broken engine cradle.