repacking rear gears

[63Stepside]

Greetings all. As you can tell from my user name I'm the proud owner of a 1963 Chevy Stepside. In a effort to extract the best possible gas mileage from my straight six I plan to install the most fuel efficent rear end set up. My question is: Can you pack 3.07 or 3.38 gears into a transfer case that once housed 4.11 gears? If so,It will cost $200 for a new ring and pinion and $300 to install. I would much rather go this route than buying a 40-45 year old complete rear end and installing it myself. If you can offer some advise please email me at andy_824@windstream.net (that's an underscore between andy and 824)