This was discussed on a thread under mods.
As i mentioned there (in the post)- running the bars across that much expanse can be a problem- and I used foam supports in the middle when I did it on a slide-in pop-up I had. Also- generally, both Yakima and Thule make bars that, in the past, only went to 60".
Another option would be quick and easys-which would allow you to use conduit or a 1x2 or 2x4 for your "bar". You can get conduit in any length you want and if you build up the fittings with tape- you can get the yakima accessories mounted.
Quik-N-Easy Raingutter Mounts - Rack-It
ps- I just checked and Yakima DOES make 78 " bars- though that might still cut it close.