Driveline polarity

I've had this car for only 40 years...why should I know this?#-o It's embarrassing........

While waiting for my new slip yoke from Brewer and company (Good People!) it occurred to me that I don't know if my driveline is correct to start with. I put it together so long ago, I don't know the answer to this question:

My driveline is not a single diameter tube. Does the reduced diameter section of the driveline fit on the slip yoke or the pinion yoke? The smaller section has been on the pinion yoke end for years..including several years of 1/4-mile use with the car running in the low 12's. Never broke the driveline...just other stuff.

Not knowing which direction it should go, or not remembering why it's the way it is....I should ask.

And so I did.......

Thanks!