Since you plan to work on your boat
and have a trailer provided, you're really looking for DIY advice?
The boat arrived there on a trailer, presumably, so all you have to do is "undo" how it got there!
An old fiberglass sailboat is likely to be overbuilt, heavy, and probably even withstood a marginal storage arrangement.
If you can get another jackstand, and using two 2x10x10s (sizes depending), jack up the opposite (non-jackstand) side. Then you can slide the trailer most of the way under it. Use the newly-released jack stands to support the new 2x10s. Measure first to be sure that the 2x10s will clear the trailer's bunks when lowered!
If that's not possible, place a stack of smaller wood pieces to knock out from under the boat when finished lowering. Repeat for the stern.
I'm lucky to have a sturdy oak branch overhead, but in your case you'd need to rent a chain hoist if the boat's really heavy. A cable-winch won't be safe enough IMHO.
Good luck—and hey—take some pictures!