You should ask the owner of this tractor for a walk-thorough of its controls...he'll be able to tell you the eccentricities of THAT TRACTOR...
John Deere Model 60 had live PTO, which is independent of the tractor speed, but
is controlled with the tractor clutch pedal, if you press the clutch half way, the tractor stops moving, but the PTO still runs, pushing the clutch all the way stops the PTO as well.
Since the PTO is independent of the ground speed throttle up so that the mower is NOT stalling the tractor when mowing.
John Deere Model 60 has 6 forward speeds...pick a speed that doesn't stall the tractor when mowing, (start off in 1st or 2nd, and see how things go) make sure whatever speed you choose, you are not going so fast as to KILL yourself...remember if you come off this tractor the PTO will NOT stop the bushhog...
If you have never been in this field before...GO SLOW!!!...The higher the grass, the slower you go...It may take longer, but you will go home at the end of the day...