Is the '76 a wet or dry clutch system? I have a late 60's dry clutch JD 450, and it is VERY high maintenance. Mostly, because I only use it around the house. Honestly, I don't think it would hold up to make a living with, so it's a house tractor. The engine was redone just before I got it 15 years ago. It's still very strong, efficient, and holds excellent oil pressure all the way through the heat range. I can leave it sitting for a year, and it starts immediately when I push the starter. The hydraulics are strong, and I enjoy the small working radius of a small loader.
The issue is with the dry clutches. I've spent around $2000 twice in the last few years replacing them because of sticking, rusting - simply not working. Older 450's were pront to frame rail cracking, and those have been re-done on my loader as well. The right side clutch/steering brake lever hasn't worked in at least 3 years. I still use it with only 1 wheel steering, but it is a lot slower that way. Avoid the dry clutch models at all costs. A friend of mine hasn't used his on the job for years because of the high maintenance of the clutches.