I don't know for sure on the 68 vs. 72 line sizes, but GM brake lines are almost always 3/16" or 1/4" tubing.
The 3/16" lines typically have 3/8"-24 inverted flare nuts. And the 1/4" lines typically have 7/16"-24 inverted flare nuts. However, some locations have special over-size flare nuts to prevent lines from being accidentally connected to the wrong ports on things like master cylinders & proportioning valves.
With that said, I'm going to guess the hard lines on your truck are 3/16" diameter with 3/8" male threads on the inverted flare nuts. And you need adapters to connect them to flex hoses that have the larger 7/16" female threads. If that's the case, look for Weatherhead #7818, Dorman 785-432, Edelmann 258340 or other equivalent adapters like this -->
http://www.oreillyauto.com/site/c/de...8340/N1415.oap
These
should be available locally if you have a good auto parts store that's willing to look things up by part number (and not just year/make/model).