Dan,
That sounds like a really complicated way to step a mast.
Here is how you should do it.
Put the mast on the ball and let the mast rest on the rear beam. If you already have your rudders and tiller on, the mast on the tiller bar may bend it permanently, so you can use something else to rest the mast on in the horizontal postion, i.e., a milk crate, a fender, a box, etc.
Now just get on the boat, ascertaining that the bow is tied to the trailer and the trailer is hitched to the car (or someone is standing on the trailer tongue)
Go back by the rear beam and lift the mast up in one swoop, much like the clean and jerk in weight lifting.
Bingo, no climbing ladders.., or falling off them.
Good luck,
Rick