Here's the figures.
With the 8.3 and 3.31 ratio my top speed would be 59 mph.
Brownie box .82 overdrive raises speed to 70.
Not too bad but not worth the effort IMO.
I don't need to do 80 mph but that's the top speed I'm shooting for making cruising at 65 possible without the engine being wound out.
An Allison 3000 series transmission has 6 speeds with both 5th and 6th being OD. The problem is controlling it and $$$$$ to buy one.
I do recall looking at a truck once that had a mechanicaly controlled DT466 and I believe an Allison 3000. I may look into this.