In 1993 I drove my worn out '63 convertible (6 cylinder w/2 speed auto with 3.5 rear) from Madison Wisconsin to Seattle. 2,000 miles. Took about 5 days as we were sight seeing as well. Took I90 and I94 all the way. Only problem is the water pump started leaking badly in Spokane. Swapped it out in the motel parking lot in the morning and continued on.

Lots of great things to see on that route and August is a great time to go.