If your 65 Valiant has a "3 on the tree" manual trans, my guess is that it has a 3.23 rear end ratio.

Get under the car and look for the ratio number stamped into the housing flange.

Those 3 speeds do have a low first gear ratio so you could most likely go with a 2.93 rear ratio without much problem. A 2.76 ratio would be tough on the 9 inch clutch.

Just as a ref, I could sell you a 2.93 SBP 7 1/4 rear end assembly and throw-in a couple of SBP 14 inch wheels for right around $100.00 plus shipping. This is the least expensive way to get the improved freeway milage you are after.
Drop me a PM if interested.

There is no reliable reference for what gears your 40 year old car may have now...

If you are going to switch trannies... go to a box with more gears ( 4 or 5 speed)

The easiest way to change final drive ratios is to get bigger tires.

I had one in an 87 Dakota; it felt and shifted like a truck tranny, I wouldn't want it in a car. I don't know if the later ones are any better.