I say go with the 454. Your 355 or 350 is not getting super great mpgs anyway. It is unlikely that overdrive will come into play pulling wagons around the farm. The deep torque at low rpms will.
My 355/350 with 3.08 gears X2 peg leg still only got about 10mpgs on the highway. So, 2x the torque and about the same mpgs? Seems good to me. In fact, I had a plan in place, but I did not get to the big block fast enough.
