I just put a hack on a DR650. (I'm still sorting it out)
About gearing. I went down one tooth on my front sprocket a while ago (to help w/ lower speed offroad riding). I'm glad I did it, it works for the hack as well. Front sprockets are fairly cheap, you might want to order a stock size and one tooth smaller. Then you can swap them and see which one you like better. If you aren't going on the highway, probably the one tooth smaller is the way to go.