I would suggest that you do both. Buy a lower-mileage M47 (or really whatever you can find) and put it in. Then put the bearings in the one that is currently in your car. A swap is pretty straightforward and on the second time, it'll only take a couple of hours to do. You should spend in the neighborhood of $500 for a used one which isn't the worst. That way you have the car.

One thing you can do to prolong the life of your trans is to get it overfilled (if you didn't know about this) and change the fluid. Obviously do those two things in reverse: Change the fluid and make sure you ram as much in there as you can. With a rumbling in 5th you still might have quite a few miles left. Typically they howl in 3rd. In fact, most of them make some sort of whirr or howl in 3rd gear, but if it gets loud, the rumbling in 5th gear around 50-65mph somewhere gets very loud.

Do you live near Chicago?
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1992 940 wagon, very low miles
as well as others.