Got some options for ya.
Lock up 545, 2300 rpm 57mph
Lock up 545, 2800 rpm 69mph
Can also use the gasser manual 3052, 1:1 5th, same speed as above. And I have one, it shifts very nice.
3053a, 2300 rpm 72mph
3053a 2800 rpm 88mph.... that might be a little fast....
Here is a link to the gear calculator, very handy for figuring gear ratios.
Easiest thing to do would be swap a lock up tranny in, and bump the rpms some. But tranny would cost ya. We may be able to get a good deal on one.
Otherwise, sell the one you have, and stick in a manual. Lot more work this way, but selling the auto should cover the cost of swapping. I have to look up the threads on parts needed to install spicer tranny behind the cummins.