Take a look at the route thats shown on the 2 Great Parks North maps. I started at the opposite end than you would be last year in Jasper and rode down to Wyoming and the scenery was spectacular.
If you start in Billings, make your way to Missoula (I think there are a number of attractive options for that) and then pick up the GPN route. It goes thru Glacier NP / Waterton NP, Southern BC, then back over into Alberta and up the rockies. Once you get into Lake Louise you pick up the Ice Fields Parkway which is just incredible.
The 2 GPN maps total about 750 miles + whatever you add between Billings and Missoula to make your distance what you want. But the clock is ticking, as others have mentioned GTS closes for road work in mid-Sept and the end of September is getting pretty close to winter in Jasper.
I believe ACA prefers that pics not be attached directly to a post, if you want to see 3 or 4 of my best from my trip send me an email at firstname.lastname@example.org