If you look on the map, 151 is almost a straight shot from Madison to Fondulac, however it's like a freeway, at least at the north end (I drove part of that on Saturday). So that's no good.
You'll probably just going to have to map it town to town, unless someone else has ridden it.
151 is definitely out. I'm hoping someone has ridden this route and can advise. I have the Bicycling Federation of Wisconsin maps, so I can map out the ride that way, worst case. They have roads outlined in various colors depending on "bike friendliness". The problem is that the maps are not always current.