One option for large wrenches is a stubby "knock wrench". Search for that on ebay. It has a pad you hit with a hammer. Especially good in tight spaces. We used one at a former company to take the top off a large valve, w/ one guy standing on top swinging a sledgehammer like a golf club to hit the big knock wrench. I have only seen as a box wrench, so might need to cut a slot to get it over the shaft.