I don't have any specific route advice...just pick out the drainages that allow a gradual climb out of McCall, and then a gradual descent into Johnson Creek. I know you're worried about climb performance, but give a lot of thought to your descent profile as well.
The close quarters of landing in a canyon doesn't mesh well with coming in too high. You really don't want to be 3000 feet AGL when you first see the field. There's plenty of room to land without having to do a turn over Yellow Pine providing you come in low enough to begin with.
When it comes to fuel, don't calculate straight-line distances and times...it doesn't work like that in Idaho. It takes more fuel to fly the big climbs and circuitous routs required. Lean for density altitude, which this time of year usually means leaning once you've climbed into the airplane. Being light is good, but Johnson Creek is PLENTY long...don't land there with 1/4 tanks thinking you need to be ultra-light, 'cause you're going to need some fuel to get out. Remember that McCall and Cascade frequently get fogged in, and you probably won't know that until you're airborn. Have enough fuel to go to plan B or C.
Probably the most important thing is to leave early. You can land at Johnson Creek any hour of the day, but the air is only stable and smooth BEFORE about 10 AM. You'll enjoy flying close to the rocks and trees a whole lot more in smooth air, especially your first time in.
Upon departure you'll enjoy climbing away from the rocks and trees in cool air a whole lot more than in warm air. If you don't have a combustion engine heater that might mean lighting your engine before it's as warm as you'd like it to be...do it anyway. The extra wear and tear of starting a cold engine is nothing compared to the wear and tear of hitting trees on take off.
Make sure your wind screen is as clean as it can be, inside and out. Early flying in canyons means low angle sun on the glass, which will be blinding if it's not uber-clean. Even with clean glass, there are places you don't want to be, like turing into the sun with a dark canyon wall in front of you...as soon as you do you'll be on instruments only, which sucks while down deep in a canyon. Better to avoid it by recognizing when it will happen and then flying a different approach or departure.
Keep your lights on all the time while flying, fly the correct side of the canyon, and practice good radio discipline...something sorely lacking in the Idaho backcountry. Position reports are good at recognizable bottle necks where airplanes are close to the ground and in tight quarters, but useless noise the rest of the time. If you can't say exactly where you are in a manner that other pilots will instantly recognize, don't say anything at all. Nobody cares what your tail number is..."Brown Cessna five miles downstream Johnson Creek, Landing" tells people all they need to know and doesn't waste airtime. If people are talking breakfast plans on 122.9, don't be afraid to tell them to go one off, regardless of who they are.
Just keep your brain engaged and you'll do fine. Have fun!