I have a guess that your planned trip has already happened, but I have run the Hidden. It is an amazing river with some …amazing… portages. You can run the first mile or so out of the glacier snout, but then the bottom falls out … waterfalls on to rocks and heinousness, etc. The bushwhacking around this section is quite legendary. Eventually it levels back out and you can float class II water, but shortly it picks up again in to a class III+ section with some powerful water. Remember you’ll likely be traveling pretty heavy and this is a bad place to realize how maneuverable your loaded boat is not (first-hand on that one). After the mini-gorge, lots of class II miles through an incredibly scenic valley. Eventually, the river picks up again. It will become fun class II, then fun class III-, then dodgy class III, and then you will come to a single defined rapid that has a powerful (mostly) river-wide hole. I walked this, though I know it has been run. Not far after this class IV or so rapid you will come upon a very tight mini-canyon. The whitewater is easy, however it’s basically swirly eddy water that goes undercut on river right. Bear trail on the left if you aren’t feeling it. At least one party (permanently) lost their paddle after a swim in this section. After the short, tight mini-canyon, the river mellows back out and is bumpy but not difficult to the Chulitna.
Getting off the Buckskin might be an adventure. We went middle and ended up skiers right off the snout, however those places are always changing and we felt like we picked a pretty lucky line.
The Hidden is an adventure … both to get to and to run. Don’t take the western side of the Chulitna drainages lightly, there have been numerous epics over the years.
The rangers at the Talkeetna Ranger Station may be fairly knowledgeable about these trips - 907-733-2231.