I don't know what happened to my post between Tom's, but the answer was that i have limited room and length with engine under the cab, so the 3.9 liter 4BT (essentially a 6BT with 2 middle cylinders removed) would be a better fit. Then use an OD trans behind that, either the T400 in the truck with a gear vendors type add-on OD, or a 47RE, or Allison. My F-650 has 5.9 Cummins with Allison, so a similar set-up with the 4BT. Here is the pic that was supposed to be attached showing the engine location and why space is tight. Under construction pic, but it shows how tight the engine cover is over engine and why length of the V8 is needed:
