I think there is normally about 1 inche of play. It should never bottom out. The sliding part of the yoke is about 3 inches long.

Let's review:
1. B310, remove auto transmission
2. Fit standard trans from a B210 into B310
3. The B210 tailshaft had larger flange at rear than B310 diff -- maybe to fit the 120Y-only B/W diff? In any case, a different or modified shaft is called for.

*The 56 and 60 transmissions are the same length (as far as we've seen)

*The automatic and standard B310s use the same differential (H150).

Premise: a standard B310 standard trans tailshaft should complete the swap.

You found a shaft at the wreckers that was already out of the car. But it was marked "Sunny". So it should be from a B310.

It all sounds right. But it doesn't quite fit. I would go back and find one from the back flange face to the "stopper" part of the front, and get one a that's a bit shorter.

It should be 49 inches. How long is the one you have (from back flange face to face of "stopper")?

By the way, it's not really a "stopper" -- it's never supposed to go in that far. The cup on the end of the trans is a "dust cover" according to my B310 factory manual. This design just helps keep most road dust from getting up to the seal.