I'd have the driveshaft balance checked too. When I installed the Dana in my Dart several years ago I had a 1350 driveshaft built by a very reputable company in Michigan. The shaft was well over $400. I chased a vibration for years, nothing I tried worked. I was rebuilding my 727 and the shaft was out so I took it to a little local shop to have the balance checked. It was way out of, now it runs nice and smooth.
