I have 4 4BT engines in the yard and have to think they would be so much easier to fit under the hood as the 6BT is so tight. But then the horsepower/torque bug bites me.....

You are getting some very good MPG numbers there. My 6BT trucks have never got over 19 and usually 15 running around. All of mine are autos so that does not help. I just got a 94 4x4 and dumped the exhaust ahead of the right rear tire and the sound when the converter locks up is sooooo sweet.