Three on the tree with a 318?

If you want to convert to a floor shift, look at a shifter like Hurst, which comes with all of the necessary linkages. A lot of the Valiants and even quite a few 340 Dusters / Demons came from the factory with a 3 speed manual floor shifter. If you want to stay with original factory parts, they are still out there. Your 67, if it's still stock, will most likely have the 7.25" rear end with the /6 engine. It has smaller axle tubes than the 8.25" and has 9 rear cover bolts as opposed to 10 bolts for the 8.25".