For years all the U.S. auto builders used the Borg Warner OD unit. It was a 30% OD and the standard rear gear used by ford, chevy, studebaker etc. was a 4.11. This gives a 2.88 in OD which is an ideal highway gear. In underdrive the 4.11 will let even a short stroke, low torque, high winder pull good. Ones fuel mileage with these old units depended entirely on the driving habits of the operator. The govenor would let the OD engage at just under 30 mph and if used like this even big V8's would get respectible mileage. With that said a 3.23 might be a little shallow but give it a try.
