I used a 1,000 lb. cable come-along to lift the fiberglass shell off of my '80 Toyota 4x4 longbed. It was upholstered in shag carpet (remember, the 80's ;)), and had curtains and and a pop-up sunroof, so was plenty heavy. I had to stand on a step ladder, but it wasn't hard, just a little awkward. If it's a plain aluminum shell without add-ons, it'd be a snap.
(on edit, the length of rope you'd have to pull thru with a block & tackle would require a lot longer travel than most door openers, unless you're going to just lift it a few inches. Just taking the slack & stretch out of the rope will probably use up a foot or more of travel.)
Jim, "Mo' coffee!"
