After cutting the floor joists to length and hauling the joists to the site, we drilled the holes for the eyebolts into the tree trunks. We basically eyeballed their location to place them as symmetrically as we could. We had to use a pretty large lever to twist them all the way into the tree. After they were in, we determined how much chain we'd need to reach from the platform to the eyebolt, and cut the chain to length.
We next start hanging the joists. It was pretty exciting sitting 16 feet up on a 4" X 10", trying to hammer in the hangers at a weird angle. Luckily we all survived, and made pretty quick work of it. We left one joist out, knowing that we'd frame in a trap door spot later.
Since the platform was 16' X 8', we didn't have to trim any of our plywood subfloor. We quickly screwed it down, stiffening the platform significantly.
Then the moment of truth came, and we removed the temporary supports from under the main beams. I was worried that the platform would swing and rub against one of the trunks, which wouldn't have been a fun problem to solve. Luckily the platform hung centered between the trunks. We adjusted the turnbuckles to try to level the platform, but it appears we'll have to re-cut some chain in order to level it out correctly.
When up on the platform it moves and sways more than I expected. I suspect it won't sway as much once we add the walls and roof.
I ended up spending the night on the platform that evening. It was amazing to hear all the nocturnal activity around me. The tree is located near a creek and there seemed to be a small army of raccoons working the creek banks. There was a possible UFO sighting, but I'm not supposed to talk about that.
I hope to make a trip back in the spring to see how the platform weathered. Hopefully the tree is still standing, the platform hasn't warped, and it otherwise survived the winter intact.