Compromise May Be In Order
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The most direct all-Interstate route from Toronto to Los Angeles would take at least four and a half days, and while it does go through Colorado it doesn't follow what used to be US-66 at all and it does go around Chicago. On the other hand, with only six days to make the trip, you don't have a lot of time for major detours.
What I'd suggest then is to head from Lansing MI down to Indianapolis using I-69 and pick up I-70 to St. Louis. From there to Los Angeles, I-44/I-40/I-15 roughly follow the old US-66 alignment. While that old route has been decommissioned, the physical roadbed is still there, now marked as various state route 66's, 'Historic Route 66', or other designations. You won't be able to drive long stretches of the old route but by judiciously picking your spots such as in the Ozarks of southwestern Missouri or the high desert of northern Arizona, you can get a feel for what the old road was like without burning up too much extra time. Also along that route are national parks such as Petrified Forest and the Grand Canyon in Arizona and Mojave National Preserve in California.
AZBuck