From the factory, 1967 Mustangs used the 1/4" size pressure hose port. The 5/16" was used starting in 1968.
I know, I know - everybody says they came either way depending on when they were built. The fact is, the only sources that say this are aftermarket catalog listings. The Ford dealer parts books shows only one style control valve and pressure hose for 1967 Mustangs and no mid-year changeover. True, a lot of 1967 Mustangs now have 5/16" port control valves on them, but a lot also have Maverick and Granada valves on them too.
If you use the 1967 valve with the 1/4" port, use a 1967 pressure hose. If you use the 1968 and later valve, then use a 1968 pressure hose. The 1968 hose is longer, anyway, and is easier to route around headers and such.