I would definitely take I-5, and not I-405. Northbound traffic in the morning on I-5 is heavy enough, but once past the I-90 junction (in central Seattle) the traffic diminishes greatly, and it's a "speed limit" drive from there to the Boeing plant.

I-405 on the other hand tends to have a lot of commuters going in both directions, with the Renton Boeing plants (737s) as well as all the tech employers on the east side of Lake Washington making for commuting patterns that go both ways.

Note that you want to take WA Hwy 526 (the "Boeing Freeway") from I-5 to the plant (signposted "Mukilteo - Whidbey Island Ferry) which leaves I-5 several miles shy of Everett proper.

In the morning I would allow around 45-55 min. all in from the airport area to the Future of Flight center - 20 min. to downtown, then 25 to the plant, with a 10 min. congestion cushion. Note the first tour doesn't start until 8:30 (and in fact the Boeing plant itself has some early start times) so there's no need to go super early.