I'm havin' a hard time visualising what you are doing.
Some advice. Don't ever ride on the thing.
Design for about 3 times the expected load, and take "wench" specs with a huge amount of "salt." If it's built by someone with names like Northern something, Harbor something, I would triple the figure on the winch.
Consider some sort of ratchet brake / safety anti fall mechanism. The ones on the two post hoist are pretty simple. A great big ratchet pawl which "clank clank clank" engages every slot on the way up so you can HEAR them
