You might be thinking about this too much. Cavebull is exactly right. To reword his comment, just find out at what RPM your engine delivers the highest fuel efficiency, draw out your driveline from engine to tire and pick the setup that puts your engine at that RPM when you're going 70mph or whatever your average cruising speed is.
You can use this formula (all unit conversions included) to figure out what your engine RPM will be:
(mph) x 336.1 x (diff. ratio) x (trans. ratio) / D
where D is your tire diameter in inches
diff. ratio is your 4.1 ring to pinion ratio in your rear diff.
and trans. ratio is your 4th gear ratio (1:1 for SM420, so you can leave it out of the equation)
I haven't looked it up but I'd shoot for around 1800-2200 RPM for best fuel economy.
