I lived and biked in Wisconsin for 8 years but somehow never traveled where you're looking to. So I sent an email out on a Dane County, Wisconsin based email list and here is the only reply so far:
"There is wide range of tolerance among cyclists re: tolerance for high speed and high volume traffic. I'd consider the paved shoulder on Hwy 8 too narrow for peace of mind while biking along with high-speed traffic, including a fair amount of trucks. Hwy 64's shoulder is not wider and may be a little narrower.
I have a nice set of DOT county maps that show numerous routes parallel to those two major northern highways, beginning 1mile south of U.S. 8. They are a mix of county highways and town roads. Most of the town roads are paved (I'm not sure if the DOT map(s) show road surface type, but he/she can inquire locally), and due to the paucity of traffic, cyclists can much better enjoy the scenic streams, lakes, forests and pastures
along the way.
There are some places where an east-bound road stops where there is no stream crossing or for some non-obvious reason, but in every case there are roads that jog south 1/2 mile to one mile, to get you going east again. He/she is going to have to gradually head south anyway to get to Manitowoc, (far to the south of even Hwy 64) and those little detours appear not to add up to more than an additional 20 miles over the slightly more direct major highways. Local roads are also a good to avoid major traffic messes in Wausau and Green Bay, with good avoidance routes to the south of both those cities.
Unless he/she loves being buffeted by the air curtain of trucks roaring by at 65 mph, that traveler will have a much more relaxing trek by avoiding those two major east-west highways."
I hope this is in someway helpful.
-Martin