What is the best way to get an overdrive auto in a 67 Barracuda?

I can assure you the numbers I gave are pretty close, my rear tires are 245/45/17 which is around a 26 inch tall tire. Cruising down the road it's at 36 not 26 at 55, maybe going 60 but no faster, I have mini vans passing me for pete sake. It has been a couple months since I drove the car but I know the rpms are over 3000 at rural highway speed. If I was at 2600 rpm I would not be looking at overdrives. I would be tickled if my rpms were 2600 cruising the country. Unless I have an imposter for a rear gear, I'm almost certain it's a 355, my dart had a 440 with 410 rear with the same tires and that was like driving a tractor down the road.