Probably 12 or so is about the best to hope for anything with a 4.11 or near gearing. I used an old three speed overdrive and bumped it up to 16-18(depends on load and driving) and am happy as a cat in the milk barn. The nice part is in OD the final ratio is 2.88 since the old Borgwarner OD's are 30%, and you still have the underdrive 4.11's for trailer toting or heavy loads. It's a direct bolt in just needing a lockout cable and a couple wires--didn't use the kickdown switch. Did have to reconfigure the shifter rods a tad but the driveshaft fit. The price of the old 3 spd OD's has come way down because everyone is going to the 400R4 or other automatic OD's or to the T5 trans.
