Jim
Welcome to Stovebolt! When you have a chance you can do a search on this forum and there are many options that you can read about.
Here is a link to a tech tip that I wrote a few months ago that will keep your existing driveline and change the gears to 3:55. This will allow you to cruise the highways all day long.
3:55 Tech Tip This option will cost about $850 with the rebuild kit and you will have a better-than-new setup ready to go.
This is the setup that I have in my '51 with the stock SM420 transmission.
