I have thought about this same issue with my 69 T50, 20K, and not really decided to do anything as for me it really is not worth the trouble. I don't drive it enough to get too excited.
However, your deal sounds different so you gotta do what you gotta to do.
Your numbers show that you if you start out now in 2H, your shift point is 16 mph. If you go all the way to 4.56 and start in 1H you are below your current 2H start ratio, shifting at 12 mph so the 4.56 should pull a normal start better, just one more shift to hit the top gear. And you still have one choice below that for hard starts (which is ~30% higher than your current low-low, you lose one ratio on the bottom). If that doesn't worry you, then the low end looks OK to me.
The splits on the gears will be wider so the engine will need to make that up with a wider power band than you need now. The top shift split with the 4.56 is 12 mph, with 6.50 it is only 8 1/2 mph, I don't know what that translates into rpm difference. (I would drive the truck at a steady speed and shift from 4H to 5H, noting the rpms in the two gears, then compare that to the power curve for your engine. Comparing the rpm difference to the difference of the current and proposed rear end ratios. That should tell you if you have enough power to pull that wider split.) If your power band is wide enough, you could use any of the ratios and be fine.
If you go as far as 4.56, the engine will be in the power mode more, running richer, and fuel mileage might actually decrease. If you retune the carb, you might get that back.
If you run the truck for long periods in a lower gear, the trans will run hotter, same for the diff. Will that be a problem? I have no clue but I would want to cruise in HH if I was on a cross country trip. 4.56 might be too high for that.
Your clutch mileage will vary.
