sounds like a great tow rig.........
i'm not much of an engine builder, and my approach would be alittle different because of it.
i'd 1st do what was required to build the engine that developed the power/torque that i wanted in the most dependable way. Then chose a transmission, can't go wrong with a low'ish 1st geared overdrive in a tow-rig. Then chose the wheels/tire (diameter) and lastly select the gear ratio that would put my engine RPM's in the optimum range while at my intended highway MPH.
just my .02
