A833 Transmission

Too late to measure the driveshaft for correct installed length.
listed for a 71/72 demon for a 340/ manual trans, I have
pn 2883760, which is
48.96x2.75
But that yoke I have never seen in any A-body .
Unsupported like that, Floppin' around to some degree is normal.
But if the input gear moves in the fore and aft plane on a straight push-pull,more than say ~.002inch, that is a worn input bearing. Worn far enough to warrant replacement? Not to me,lol.

Consider how that floppiness is created. The front and rear bearings are the fulcrums, and the output shaft is about a 3:1 lever. So if the back-end of the M/S moves say .32 from min to max, then the nose of the M/S moves ~.107.
Which moves the input gear also .107. But the front ratio is more like 4:1, so the tip of the input could move ~.428.. Of course the input shaft is restrained physically by the retainer... so you might not see the whole .428inch, but to you it looks floppy. This test, IMO, means very little.

But the real test is to stick that elephant trunk into the back of the trans, and wobble it in the same way. The rear bushing is maybe 3 inches long, and a close fit on the yoke, so there should be very very little wobble there.

And, I would inspect the rear mount very closely for a reason not to throw it away.
One more; inspect the oil that came out of the trans. Put it into a shallow pan like a frying pan with a shiny bottom. Shine a light in it and swirl it around with a tool. You might see a tiny amount of brass glittering back at you, but no silver glitter. If you see silver, it's tear-down time. If you don't see silver, chances are there is nothing wrong with the gears. If you can't find the source of the thumping externally,like in the yoke or mount, that is the only reason I would tear it down.
Well unless you told me it was:
grinding on the shifts, jumping outta gear, or the bearings were singing,lol.