I just did this last month, real simple. Like they said, swap the yoke and cover and your set.
Denny I think there was a difference in the synchros only, but the gears, countershaft, blah blah blah, should all be the same. They did make a few goofy splined sets in military and big trucks that may require a different seal or yoke. I wouldnt know exactly as I havent seen any of them.
Theres a few rarities out there and they even used some of these trannys in odd applications far past 1968, up into the early 80s I believe.
You can identify the date of your tranny by looking at the code stamped on the right hand side above the inspection cover. Usually has a serial # (5-7ish #s) and an ID #, usually a 4-5 letter/# sequence. First should be a letter indicating month (A-jan, B-Feb, etc...),Next number is the day of manufacture and the last 2 are the year of manufacture. Mine was something like D163, April 1st 1963.
Hope some of that helps. Jeff
