I was last up the alcan about 5 years ago. So my info is dated. But If memory serves, I don't thhink there is any stretch longer than 100 miles for fuel. Getting fuel is not the problem, getting service could be.
The Alcan is not a wagon trail it is 2 lane black top, I have seen much worse in the lower 48.
Rather than extra gas, I would be much more concerned with tires, belts and hoses,
I would take tools to fix stuff, make sure tires etc are in good shape, and bring duct tape.
If a radiator hose blows you can duct tape it, I would bring extra hose clamps, some bailing wire that sort of thing.
As in if something goes south you have enough stuff to jerry rig things together
for a 100 miles or so