I did the same thing you are trying to do, but with a 55 1st series. Mine has an open, one piece drive shaft. I don't really understand what all of your settup is, but as a newbee, such as myself, I figure you may benefit from one piece of information that was vital to my change over.... I went to a junkyard where they keep all the drive shafts in an old school bus. I had measured what I needed for drive shaft length when I got the 4 speed in the truck. I noticed differences in the u joints on different shafts. The junk yard elmer told me that at NAPA you can get U joints to convert one size U joint yoke to another, and therefore use any driveshaft of the right length. And, it worked for me.
