The reason many of the respondents mentioned the heavy truck traffic on the Cassiar is not only the power-line project, but also huge, double-trailer trucks traveling at high speed from a mine north of Whitehorse to a ship-loading dock in Stewart. Their shop/overnight yard is located across the street from the Bear River RV Park and the trucks begin leaving there at 3:30 AM. However, I read that the mine had either closed or reduced production and staff, so that may not be a problem this year.
Sue T. would have more info on that subject.
And I'll put my vote in for using the Cassiar on one leg of your trip. IMHO, it is more scenic than the Alaska Highway segments between Dawson Creek and Ft. Nelson or between Watson Lake and Teslin.