Okay, I misunderstood what your current driveshaft looks like. Most of what I've been worried about should not be a problem. What you describe is basically what I remember having on later trucks.
The question at this point is only how do you connect the driveshaft to the transmission, and the answer is either to modify the existing front shaft, or to locate one from a truck that had a 3-speed or an automatic. Automatic is the surest bet, and I'm pretty sure a Powerglide and Turbo 350 are the same length as the Saginaw 4-speed, so that should be an easy fix.
