Cool, Soldotna, great place. Did all my annuals and maintenance there at PASX. Which airstrip did you buy into?
As for route, I have a C-172 (56) TD though it has a 180 hp conversion, but have made the trip in essentially the same plane with the stock 145 plant, it was just a longer trip and have done the route high, low and in the middle, depending on what the weather allowed. The time of year you are proposing there should be no problem with getting fuel since you'll be in the midst of tourist season and everything will be open and you will have lots of light to fly in. As previously stated, plan on flying the Alaska highway you're first trip, there is an Alaska hwy sectional available, though I would really recommend getting foreflight, especially if you are flying solo. The huge relief from finding, folding sectionals and trying to look up information in supplements is remarkable and in the end, not much more expensive than buying all the charts/supplements when you figure the foreflight subscription is good for a year.
CANPASS is great, but I would only buy it if you are planning to cross into Canada more than once this year, in all my trips, Canadian customs has never been an issue, and in fact, have only once actually seen a Canadian customs officer, and she was pretty cute

. Much more often I have contacted them on the customs phone after landing and they say "welcome to Canada, have a nice stay".
The furthest East I have crossed is Cut Bank, MT to Lethbridge. Cut Bank is a good stop, nice airport with internet access, a courtesy car and a place to crash if needed. Clearing customs at Lethbridge was a phone call. Whitecourt CYZU is a good stop, but so are many of the places along the Hwy, it really depends on the time you want to spend, what you want to see and, of course, the weather.
Coming back into the US in Alaska on the Hwy, is trickier. There is customs at Northway, but they don't man the office there anymore, so they expect a fairly accurate arrival time or they get testy. Last I heard they close at 3:30 Alaska time, they may extend that to 4:00 during the summer, but I would definitely check that. No fuel there anymore, so you will have to go up the road to Tok; great place to eat and stay, just across the Hwy from the airport.
I'm guessing you already know about eApis, customs sticker, passport, survival gear requirements, firearm regulations you will need for the trip. If not AOPA has good sections on both Alaska and Canada under flight planning.
If you want anymore info or have questions PM me. Its always a great trip. I have always been in a time crunch and have done the trip in 3-4 days from beginning to end, about 27-30 hours of flight time total, Take you're time if you can.
Chris