I hang three sheets with a sheet of cut down 1/8" plexi and cut down 4x8' foamcore. Frame rails from Grafik Dimensions. The frame comes in at a bit over 60 pounds and I hang with two 120 pound hangers, driven into studs. Takes two adults to lift it into place and steady it while the hanger wire is slipped over the hangers.
I have thought about hanging a six sheet, but have never found the plexi large enough. Glass would be way too heavy. If you did the poster with plexi and foamcore, you could expect it to come in around 115 pounds.
Might look into matte board for a backing. It is thinner and lighter, but still sturdy. If you haven't trimmed the backing, there is likely space for some hooks beyond the first ridge. You could let the backing run over the ridge and then hook it in several spots along the top.
Just some random thoughts.
I think I have posted this before. My son next to a three sheet I had hanging in previous house: