Fermentation should be fine since your beer is going to create a LOT of CO2. However, I would be reluctant to open that fermenter for any reason because once you introduce air in there your already fermented beer won't release enough CO2 to push the oxygen out. If you're going to use that large of a vessel to ferment in I would suggest fermenting for 4 weeks and then going straight to bottling. 4 weeks should be enough time to ferment the most stubborn of beers and condition prior to bottling.
It's not ideal, but it will work. Ideal is the least amount of headspace you can get away with without risking the krausen exploding out the top.