There arn't many options for an over drive in our trucks. There was a Clark 5spd with OD, it's listed in the shop manual, and the 3spd. IMO the 305 would tear up the 3spd with out much effort. Gear Venders has a OD, but they are a little pricey. Another option would be an RPO MY6 (aka NP 440, A833) from an 82-86 truck. It looks like a passenger car 4spd. but 4th is an OD. Chrysler used there version behind everything from the the slant six to the hemi,and in everything from the Dart to 3/4 ton trucks.
Larry
