I know a guy that had a Cummins 4BT in one and a Detroit 3-53 in the other. Both 1960s IH 2WD trucks with straight axles in front.
I've been waiting for a good deal on a 4BT for years. Pricey and never seem to go cheap. 6BTs I find all over the place fairly cheap. I just passed one up for $800. Cheapest 4BT so far has been $2200 and 1500 miles away.
Nothing wrong with the 6BT. Just a fuel mileage thing. An IH with a 6BT and proper gearing can probably get 18-19 MPG, whereas a 4BT can probably get 23-25 MPG.