Tramp:
I am using the 3 speed with overdrive that dwragon is referring to. It is a nice set up that would be a bolt in swap for your 3 speed. It reduces RPM's 30% in overdrive. Here's some discussion and a site with the original manuals if you would like to read up on them:

https://www.stovebolt.com/ubbthread...;Main=67910&Number=489877#Post489877

http://speedprint.com/deves50/overdriveindex.php

regards,
Steve
New Orleans