When you're in Tok, check the weather towards Valdez. If it is good, go that way first. If it is damp, then go towards Fairbanks and maybe Valdez will improve by the time you get there. Our last two times, we headed for Valdez from Tok because it was sunny and clear and the long-term forecast was for blue skies.
Going over the Top of the World highway in the fall is spectacular. The reds are amazing. The highway is usually in better shape in the fall too, after summer maintenance is over.
Same for the Cassiar - better later in the summer.