I'm using the same '78 XT500 forks on the bike that I had on it for mock-up. They were in nice shape. I just cleaned them up, and put in new seals. I'm expecting I'll need to go with an aftermarket spring set, but I'll try these first.
For the fork gaiters I bought some Tuff County shock boots part number 69127. These are about $11 shipped on ebay. I've shortened these for use on stock XS650 forks, but they fit perfectly out of the package on the XT500 forks.
Here they are ready to go. I'm hoping to have the bike a roller by next weekend.