I'm not sure about the driveshaft, but you will love the M20. I have one in my 67 Camaro with a 3.73 gear and it has took all of the abuse my 406 can give.
I highly recommend that you have the flywheel and pressure plate balanced. When I converted my car to a 4 speed I didn't have it balanced. When I pulled my 350 and put the 406 in, I had it balanced and the difference was amazing. I had a vibration through the shifter that was pretty bad over 3500 rpm before, and now that its balanced, the vibration is completely gone. Well worth the money in my opinion.