I had my transmission rebuilt this past week by a local mechanic and he was able to use the 5th gear from an '81-'83 close ratio 5-speed in the '77-'80 wide ratio 5-speed he rebuilt.
This keeps the best of both transmissions. The superior acceleration from the early one and the taller cruise gear from the latter.
My mechanic was resistant to write up how he did this and I'm no mechanic, but he did say he assembled and lined up everything as if the tranny was in neutral and then had to drill holes in a difference place in order to make it work. I apologize for not being more helpful, but I can say this is possible because I've driven the finished product.
When I asked him to try this, he warned me their would be a large difference between 4th and 5th gear but I had driven both transmission and wasn't worried about it. After assembly, he said their was much less of a difference than he expected. I'm very happy with the setup since I get the acceleration back and I keep the lower rpm's at cruise.