Ours is a 1977. Bought it 3 years ago. We are on the Ohio river in southern Indiana . I have basically had the boat completely apart so if you have any questions just ask. I also have the owners manual if you need a copy.
Sorry for the slow response. I missed your post. Not to many San Juan 26 around. On my sj26 the tube and base are threaded 1 1/2" galvanized pipe. Should be available at Lowes, home depot, or any plumbing supply.
I'm in southern Indiana. Near Evansville. About to replace my centerboard cable next week on my San Juan 26. I gave up finding any stands. I plan on blocking up the keel with 6x6s and building supports with 2x4s and plywood to support the hull and keep it stable.
Just couple hours south of you on the Ohio. Bought our boat last year. Worked on it all winter and half the summer. Launched it 3 weeks ago. A couple of great days on the water at first but now water is like glass every time I'm near the boat.