We've done this run several times. Sorry, as I'm at work, I can't give you road numbers, but I'm sure you can look it up.
We take the motorway to Troyes (about 4 hours & a good municipal site if you want an overnight stop). We then cut across country on a diagonal, through (round) Auxerre, Nevers, towards Moulins & pick up the autoroute around Clermont Ferrand. This is a toll free road with some good scenery and you just follow it south.