Maybe someone can confirm this: some cars had a tapered driveshaft with a small u-joint on the front and a big u-joint on the rear. If this is the case then I would assume the tappered end is the tranny side.
Again, hopefully someone else can confirm this since I'm a little rusty. I think my factory driveshaft also had a tapper but I replaced it 20 years ago and don't know what I did with the shaft.