The overdrive I'm using the Borg-Warner R-10, which was a factory option on trucks starting in '55. It's an integral unit with an SM318 3 speed main transmission. With the 3.55 gears you have, an overdrive might put a 235 out of its comfortable rpm range.
If you use a '58-'62 261, a hydraulic cam should be able to be used in that block. They are solids stock, though.
Another thing you might consider is a taller tire. That will also help with getting a bit more speed with less rpm.
