Phil,
I think you are right when you say what is needed is an overdrive box, not more gears.
Assuming the 216 has enough power to pull an overdrive ratio on normal flat roads with a bit of a headwind (it certainly will be requiring a lot more shifting regardless, over the current 1:1 ratio on gentle rises) the ideal would be a box still having the current 1:1 ratio with an overdrive on top of that.
If you are going to put a non-standard box in why mess around with big heavy GMC boxes when you can have a delightful light modern overdrive box with full synchro.
Regarding the advantage of more gears. Mack produced a sales video some years back to demonstrate what gears truck operators should have for their operation. They had two identical Mack semis, fully loaded at the bottom of a very long hill climb. The only difference was one had a 5 speed box and the other a 13 speed box.
In the drag race the 5 speed truck won by miles as it maintained momentum for longer against the periods of loss of drive suffered by the continuous changes of the other truck. In the next race the 13 speed driver jumped 2 gears at a time and although he still lost he was much closer.
They then went out on to the mid-west plains with gentle rolling rises along the freeway. In this case the 13 speed truck maintained much better speeds and returned lower fuel consumption because it always had the fine adjustment of just the right gear.
An overdrive on the CMP (if the engine can pull it) is the go. Of course putting bigger diameter tyres on has the same effect. Just having more gears in our old vehicle situation is pointless.
Lang
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