Adams' Apple said: Back in the day, Joel Rosen(Motion) installed some Hone O/D units in Shark cars. Those are about the same physical size as the GV units. The amount of cutting and fitting to get that unit in was incredible. The trans tunnel had to be widened quit a bit, and the frame/crossmember modified to the hilt. In the end, the cost wasn't worth the trouble. He only did 3-4, if I remember correctly. I have a tech article on it somewhere, and it shows what all is needed to complete the install.
There IS a kit available to put an overdrive gear into the stock Muncie 4 speed. It is still a 4 speed, but 4th gear is overdrive. About $300 for the kit, last time I looked, plus the labor for the conversion, and it all fits exactly like it came from the factory...no cutting or any other modification. It involves changing the cluster gear, and the 3rd gear on the main shaft of the trans, and flipping the 3-4 shift lever over on the outside of the trans. It's a pretty neat set-up, for someone wanting to keep the stock look/trans, but have an O/D ratio in 4th. The bad part is it will not work in a BW Super T-10 trans....only the Muncies.
The next best option is to go with the Keisler, or similar swap to a 5 or 6 speed. Lot more work, but you get better ratios all around that way.