Your NH 492 is a 1996 model. It is a 9 foot 2 inch cut. In light hay the JD 830 will pull it but in heavy hay you will need the Oliver 1655. It also depends on how fast you want to mow. To keep a fairly fast ground speed in heavy hay will take more horsepower than the JD 830 has.
Had a friend that had a NH 488 haybine. He tried to run it with his WD 45 and had sickle problems all of the time. The ground speed was wrong for the PTO speed. IT was wedging the sickle full and then having to shear it packed full. He switched to his AC 185 and the sickle issues went away. This was in waist high thick timothy and heavy alfalfa.
IF your hay is thinner then you may get along with a smaller tractor but I would not risk it myself. You need a tractor with the HP to hit a thick spot and pull through without bogging down.
Also like rrlund pointed out the Oliver is a lot nicer tractor to ride across a field than the smaller JD 830 utility tractor.
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