You have done your home work some on gear ratios but just remember that it is not all simple math because with larger tires and increased height also come increased drag and the lowest possible RPM is not always the best for MPG. A heavier truck truck needs to turn more RPM cruising for best performance and MPG because you do not want to have it kick out of OD on most hills on interstate. Myself, I think 2100 to
2200 rpm at 60 with a GM SB in a 4x4 is a better compromise for power of MPG in OD.