Don-
A couple things come to mind with regards to Originals and slipping drivelines.
First of all, you'll need to figure out if the belt is slipping, or the clutch is slipping.
Since you've got a new (and correct) drive-belt, let's rule that out. Did you get the belt good and tight per the instructions in the manual?
It doesn't have to be "Gorilla tight", just good and snug per the instructions. If it's as tight as the manual suggests, I'm betting the belt is not slipping because I'm able to pull a 10" Brinly plow in 3rd gear (in loose soil) and the first thing to slip is the actual clutch-plates, not the belt.
Next I would check to see that the clutch pedal is returning to the "full up" position. Many of these Originals have been used and not maintained for so long that the bushings which hold the clutch/brake pedal are worn out and allow the assembly to bind and get stuck keeping the pedal from returning all the way up. (Hook your foot under the pedal and pull up making sure the pedal hits the pedal-stop.)
<font size="-2">the manual says you are supposed to oil that bushing, but many people didn't so they wore out.</font>
Once you've confirmed that the pedal is working properly, make sure that the clutch/brake assembly is adjusted correctly such that when the pedal is all the way up, it's not holding the clutch in an "open" state allowing it to slip.
Again, from the manual...
In my opinion, at this point you should have found your problem, but maybe there is a clutch-component that is just plain worn out/broken. Check to see that the spirol pins that hold the cluch plates are not broken. Sometimes they'll break, but bind up enough to drive the tractor until the going gets tough, then they slip so you think it's the belt or the clutch.
If everything checks out up to this point you'll want to remove the clutch to look for something that's damaged. Look at the bushing in the center of the main drive pulley (the one with the clutch material on it) if that bushing is worn out, the pulley will wobble enough that it slips on the plates. Next look at the actual clutch material, check the pressure plates to see if they're glazed over, and finally inspect the springs to see if they're not broken/damaged.
That's all I can think of at this point. Look it over and let us know what you find. I hope this has been of help to you....