Hy chevy dog, I don't want to be a downer, but are you sure it's a 409, and not a 348. According to my Canadian parts book the 409 was not offered as an option until the 80 series. Of course someone could have swapped it in there. As to the tranny, it looks like the stock transmission for that application was a Clark, either a close ratio [2653V], or a wide ratio, no mention of overdrive in either of those. Anything could have been changed in the last 45 years to keep the truck going, so get under there and look for some numbers on things, hope that helps.