Have you ever checked the teeth on the ring gear? That's your likely problem.
When you replace the clutch, plan on replacing the ring gear as well.
" Since I have to take the pan off,"
I assume by that comment that you do not own the manuals. Best get them before you waste your time trying to take the pan off.
Splitting the tractor & replacing the clutch is covered nicely in the I&T FO4 manual. (see tip # 39)
See tips 12 & 59 before you split it.
Check the ring gear & replace it if need be.
Have the flywheel resurfaced.
I'd have the clutch & p/p rebuilt. And I'd replace the t/o & pilot bearings & front transmission seal while I was in there.
Use the main shaft as your clutch alignment tool.
If you don't have a local rebuild shop, try these guys:
Automotive Manufacturers Inc
2400 N Lombardy St Richmond, VA 23220
They do mail order work. They rebuild carbs, clutches, pressure plates, water pumps, alternators, generators, starters & reline brakes.
If you prefer new parts, do NOT order them off of eBay or buy them at TSC. Get parts (and the manuals) from this site or Just8N's.