Todd, I don't have any experience with your tractor, but I have converted my cars that were older than 1994. Cars changed over in 1994, ie, all cars 94 and after had R134. Some may have converted before then. The process for the cars I have converted included changing out the drier, flushing the condenser and draining the oil from the compressor. The purpose is to get rid of as much of the old oil as possible and refill with oil that is more compatible with r134. It used to be recommended that all of the o-rings should be changed, but most shops don't do that any more. I would then add the right amount of oil, pull a vacuum, check for leaks and then charge with r134. You use about 80% of what the original r12 capacity was because the r134 will run at a higher high side pressure. The cooling is usually not as good because the r134 needs a larger condenser than r12, but it is usually adequate. Good luck, and if you need an A/C forum for advice, here is a good one:
http://www.autoacforum.com/
There are a lot of good instructions and techs who can give good advice. I buy all of my auto A/C parts from ackits who sponsors the site. They are in Phoenix, Arizona, and use the parts in their own repair shop.
