[Guy]

Our 1984 has the 4 speed. It does exactly what it is asked to do. On flat land it is very comfortable at 65 mph and 1700 rpm .We spend time in the west, and pull an enclosed approx 10k lb  trailer with a car inside. On hills it would be nice to have another gear. It is a big gap  down shifting when starting to lug on a long pull. On the positive side, it forces me to keep my rpms up and just take my time going up , and down mountains. Being a "classic", we have engine heat constraints and more rpms and  less speed is a very good thing on mountains in the 7% range. I am guessing you folks running Detroit 60s don't have the engine temperature issue. 

I just had the transmission serviced in California and the repair shop which also services Cal Fire trucks told me the 4 speeds were the bullet proof transmissions in the day for fire fighters.