If you inspect the old trans closely, you might find a manufacture date stamped on the edge of the case, at the very front of the trans. Hard to see it, but here is the date stamp on mine. Both my OD and my donor were 1953 units.
Hollander says 41-48 have the speedometer connection on right side while 49 and up have it on the left side.
The big difference over the years was the internal change from strut type to pin type synchronizers. I don't know a good way to tell what is inside a trans without dismantling it.
Hollander book says: "it is not possible for us to indicate which are pin-type and which are not, beyond the fact that pin type trans were first installed in 1951 cars. Pin type trans are identified by a slotted head bolt on extreme lower position of extension housing, by the 4 leaf clover stamped on date line on L.H. side or by "PT" stamped at rear of case on driver side."
But, neither of my 53 pin type units had any of those identifiers. Go figure:D. Wish I had the four leaf clover, sounds pretty cool.
There are forum folks that are much more knowledgeable than me on transmissions, maybe they can offer more info. You may get lucky and find that yours only really needs the bearings replaced, with no need for a donor trans.
I dealt with a place in Portland Oregon called Transmission Exchange. 503-284-0768. There was no kit available except the Overdrive gasket set and the "small parts kit" that contained bearings and such. Everything else was sold piece by piece. Cost me a little over $300 in parts.