Hi Ron,
In my experience, it is much easier to attach your lift panels with the roof upside down as you've described. I believe both ATC and FWC assemble in this manner. It's very difficult positioning your upper panel hinges in place and riveting when installing as you would be doing this upside down in the camper. Bad for blood pressure too.
So, once the panels are attached, tying them off as you suggest, and flipping the roof over and setting the roof on the camper body is the way to go. I had two people help me flip mine over. Marty, at ATC, recommended making some boards as jigs to hold the weight of the roof when your ready to attach the bottom hinge of the panel to the camper wall, unfortunately I can't tell the measurements though. Cutting two boards the same length ensures uniformity. Anyhow, much of this is in my thread which I'm including the link. One thing I also would mention, is that you can only have one end of the roof, or the other, up at a time without the fabric side liner on, or it is unstable and will fold up on you in a way it's not supposed to. Once the liner is on and attached to the top of the roof, it will hold the panels in position fore and aft. So, I hope that helps you a little bit, along with the link below.
Poky
http://www.wanderthewest.com/forum/topic/12116-new-build-1981-grandby-to-become-a-hawk/page-11
Thanks Pokey, When I installed my prototype lift panels I raised the back first. Once that was up, I clamped some boards on each side of the rear wall and sandwiched the rear lift panel in between to keep it from flopping over. That won't work when I have to install the canvas. With both ends up, maybe some diagonal braces front to rear on the inside will work.