Compare the Bluebird to the one on the right.... (1) You have no side bracing on the vertical post... To me, that is a little scary. (2 The one on the right has a supplementary brace on the top to provide additional strength to keep the lifting arm from bending. (3) Since the one on the right is adjustable, that means the additional length inside the outer shell makes it stiffer, and therefore stronger.
If the Bluebird were mine, and I was attempting to do what you have in mind; I would make a sleeve that would fit over the lifting arm, and attach a hook underneath the sleeve. You could then slide it to whatever position you desired, and hold it in that position with a chain attached to the top of the sleeve, and running back to the top of the vertical post, where it is secured by a bolt.
OR, just rent a hoist for a day..........better safe than sorry.......
Good Luck