Take a look at the Mystery Ranch backpacks - in particular, the Terraplane. This is Dana Gleason's relaunch of the Dana Design packs that were widely regarded as the pack that could make a 100 pound load carry like 50 (!) This was back in the 80s and 90s. I had a Dana Designs Terraplane, and loved it - all 7 pounds of it. I've heard the Mystery Ranch version is just as good.
A second recommendation would be the Osprey Volt 70, Ariel 75, or Aura 65 (probably not large enough) - but the maker does not rate their carrying ability at the weights you're talking about. (See the possible solution of caches or mail drops, below.)
Oregon Mouse can tell you about Deuter packs that might be appropriate. Deuter is just as good as Osprey, in my opinion. However, I've found that Osprey packs generally fit me better than Deuter; Mouse has found that Deuters work better for her. Which brings us to a critical point (which you may already know): Fit is Everything! Just because the pack gets good ratings doesn't mean it will fit you correctly. Make sure you spend time putting your load, not sandbags, into the pack before you buy it - and make sure the store will allow you to return it if you take a few short dayhikes and find out it doesn't carry right.
If you want to see a good video review of the MR Stein packs, go to "Watch Mike Hike" on You Tube. I know Mike - I taught him to backpack when he was about 14, helped him earn his Backpacking merit badge, hiked with him a bunch until about 5 years ago when I finally admitted I couldn't keep up. After doing a tour as a Navy SEAL, he thru-hiked the AT to decompress; he's now a firefighter/EMT and assistant manager at an outdoor store in Virginia Beach, VA. He's the real deal, and a really nice guy to boot. (Also take a look at his 30-second "summit of Mt. Rogers" video.)
Now, a question from someone who knows nothing about the route you're planning: do you really need to carry 55 pounds of food? Could you put out, say, 2 food caches and reduce your food load to 20 pounds between re supplies? (Mailing boxes to post offices or backpacking shops along your route would also work as well as caches.)
Finally, don't forget that you'll be carrying significantly more than 70 pounds: there's the 55 pounds of food and 15 pounds of gear you describe, which totals 70. I'm assuming that you plan to drink as you hike, so add 2 pounds for each liter of water you'll carry at one time (I'm assuming 1 liter, if you're following a river.) Then there's the weight of the pack itself - I can't imagine that an adequate suspension for that load can be found in a pack that weighs less than 5 pounds. So, your load will be closer to 80 pounds.