The little casting that bolts on the back of the transmission over the ends of the two shafts (cluster shaft and idler shaft) is an excellent repair as the two shafts are notorious for leaking. You may have to grind the head of the retaining bolt down a bit to be able to install the assembly. This is a better choice than pulling the transmission in order to replace the shafts.
A more permanent repair can be accomplished when the transmission is out of the car for overhaul. Many of the suppliers (Bratton's for one) sells the two shafts with the ends modified with an O-ring. These work very well to prevent oil leakage.
Tom Endy