For the last 5 years I have run a 23 spline O/D A-833 - I used it when I drove across country. The gear ratios for that unit are 3.09, 1.67, 1:1, .73. The first gear on this unit is really not right for a big block, but it works OK. However, when I temporarily put my original non O/D 4 speed back, I couldn't stand driving the car on the highway - it felt like a buzz saw so I put the O/D right back in!
The Passon Hemi 18 spline O/D gear ratios are: 2.66, 1.59, 1:1, .80. This makes for a much nicer gear spread, and it can take much more torque.
I realize that if you compare extra gas costs without O/D vs. the price of the O/D transmission, you will likely never save enough money with the O/D to justify the cost. However, the O/D makes me enjoy driving my car so much more that in my opinion it is worth every penny.
Good luck,
Hawk