You were listening to our conversation weren't ya?
ah, yes, sorry, I missed the point where you were saying it would be much safer. Yeah, it would be a ton safer than some of the junk I (we) have used. I would say one thing about a gantry crane that I'm sure you guys have already thought about. All of the reasonably affordable ones that I have seen, be it from Summit, Harbor Freight, Northern Tool, etc... All of them say not to move it when loaded.
I actually went to buy one of these from Harbor Freight this summer, but they were out of stock so I improvised. It's a good thing they didn't have them I think because I immediately started dreaming up 2x6 outriggers with pneumatic tired casters for back yard use, hooking up a trailer hitch to drag it around with the mower, then came back to reality and just backed the trailer up under a tree. I had big plans for it. Of course, you can't move a tree when it's loaded either. She who must be obeyed killed my other plan. Forklift!
With a trolley and chain falls you get 2D movement, and with a couple of extra guys and a smooth floor, I'd probably not have too many reservations about moving it (slowly and gently) with a couple hundred pounds of boat motor on it. With a big block or a diesel on it, I'd probably heed their warning on moving it loaded, or at least have the load close to the floor in case the momentum from pendulum motion makes it tippy. Suddenly I remember grade school and getting enough guys on the playground swing going full tilt trying to get the swings feet off the ground. haha.
I'm sure you'll enjoy it at any rate!