Sounds like a unusual setup to me. If you wanna go to a one piece driveshaft,buy one from a wrecking yard that is too long. Then take it to a good machine shop and have it shortened and balanced. At the machine shop they should ba able,to tell you what measurement to take,so they can shorten it to the proper length.I went thru this when I converted my 55 from a four speed to a 700R4 automatic.Course my driveshaft was already too long as the 700R4 was longer than the 4 sp.