So,... you have "fallen" in love with your 510 wagon, well that can only be a good thing. The five speed tranny from the series one & two Bluebird is a good choice & they are overdrive types. Another is the dogleg five speed from the 200B SSS, & these are also overdrive trannys, but a bit hard to find & they really offer no real advantage over the Bluebird one.
All of these trannys were originally fitted to L series engines & may even fit right up into your chassis with a bit of luck.