If you are building them and you want to include our trucks, the best thing would be to make the unit with an optional adapter for any Ford Diesel trans. If you must pick just one, your best option is the C6, because they are a good trans, but without the O/D you'll really suffer on fuel mileage. The T19 is a little better but still revs very high, especially with 4.10s, and could use an overdrive. If you adapt it to an E40D or ZF5, you will cover more than just the IDI which is good for you...should go all the way up to 97 at least./ Let us know more about your units. OD kits are always nice on these trucks.