If you've seriously considered a winch, that sounds like the best idea. 3 holes in the floor should be sufficient, not too bog of a deal, you can patch them when you move. Another idea may be to hook the winch very low on the wall. I can't tell if the walls are block or framed from the picture. If they are framed, you could hook to the bottom plate and have enough of a base to pull the boat in that way.
I'd also recommend finding a really heavy duty caster of some sort and pulling it in tongue 1st. Just drive straight up the alley to the spot where "someone" painted "X's" in the steet, unhook and rotate trailer 90 deg, then pull it straight in the garage (if you can fit it lengthwise). If you had two people & a fast winch, this seems like it would be quick & easy, the winch will control speed up/down the slope.
Added thought: Another way to mount the winch would be to use wood to make a frame that pushes on the inside front of the garage door opening at the floor. Basically, lay 2x4's down from the front to the back of the garage, then make a slightly more substantial crossmember in the back of the garage to mount the winch to. This would be a pain and be a bit rigged, but would allow something for the winch to mount to that is non-permanent.
Last edited by Ben; 01-24-2005 at 12:51 PM.