Pull the yoke back out, and MEASURE the depth of your splines, inside the yoke. Then, measure the total depth of the yoke. Using the first measurement, mark with a Sharpie the end of the splined area on the outside of your yoke. This should be the absolute maximum depth to which the splined output shaft inside the extension can go to in your yoke.
Now, measure the distance from the end of the output shaft to the outer edge of the seal. Take the difference between this and the depth of your yoke's splined area. See if your yoke will slide in so far as to leave no more yoke showing than this difference. If so, you've slid it home.
If you can't slide the yoke home, try turning it a few degrees, then mating the splines in again. After so many decades, these things wear in a bit, and some positions will slide home more easily than others. I had to replace my U joints a couple months ago, so this little exercise remains fresh in memory. I used some red moly grease, (NLGI Grade 2, as specified) but want to replace the red moly with Lubriplate's NLGI 2 bearing grease, which Mopar preferred when these cars were made. I'll get my chance to re-lube the yoke when I replace my rear leaf springs this Spring. If the yoke won't slide forward and out easily after a few tries, then scream at the shop that rebuilt your tranny.
After you get the yoke to go forward into the extension, see if your rear U joint will now allow you to replace the propeller shaft. It should. NOW, slide the shaft back toward the rear end until the rear U joint is nested well into the seats of your pinion yoke. If you've made it this far, all is well, tighten your nuts on the U bolts in the pinion, tidy up, then take your ride for a test drive.