Just an update on this in case anyone has a similar issue in future and finds this on Google.
I ordered a new countershaft along with everything from the intermediate plate back, including the new 5th and synchro design, thrust washer and pin, selector hub etc. It all fits together fine, except the bearing on the output shaft now needs to be changed to a smaller, thinner bearing (new type) and the case will need a custom adapter machined (or a newer style case I guess) as the original bearing is too large for the new 5th selector hub to move into position.
