I love John Deere 7700's. Have run some, but I actually learned to cut with a J.D. 6600. I'm sure back in the mid to late 1970's, some farmers either made the modifcations themselves or had someone else do it, who really knew the integrity of combines. Any structural engineering mistakes on a scale of such magnitude would never have made it past the local coffee shop or cafe.
If some public library still maintains any old issues of periodicals to some 30 years, back, a bit of research will unearth the stories in Successful Farming about an Iowa farmer who made a remarkable 300-bushel hopper on his Model 7700. Later on, he did the same thing for his 8820, this time, bringing its payload to 400 bushels. One mistake and that's all she wrote--a combine with a broken frame.
Super hopper combines are definitely not for everyone. Always consider the farm's topography as well as the combine itself. I see it as novel but still high risk. Combines today, really have adequate bin capacity and harvesting has become more mobile now, as grain carts have even found their way into the wheat fields, and bobtails have given rise to semis. With that in mind, IMHO, mega hoppers should rest on the part of transport wagons rather than the combines.
Seems like everyone is moving away from their old 4400's these days. That 7700 is somthing else. Somebody put some serious time into that sheetmetal work. How do you get 6 mph out of a 216 header? Mine starts pushing dirt in tilled ground and starts pushing stalks instead of cutting at about 4-4.5 mph. Are you running the feederhouse at a faster speed? I always run at the minimum speed like the manual says. I was told that the wobble boxes like to self destruct when run at the higher feederhouse rpm's.