Well, first I would look into whether it is french or italian threaded, because then it's "right (wrong!)" threaded. Considering it's a 1968, if it is french or italian, you probably have a right handed bracket. anyway, i had serious hell with my own peugeot fixed cup of the bottom bracket, i left every LBS in town disappointed and left the mechanics holding each other and weeping. Liquid wrench really disappointed me, it didn't help at all, as much as i doused the fixed cup. But Sheldon Brown is truly a god among men and he and Jim Langely both recommended a bench vise.

If you have a metal bench vise, you can very carefully fit the cup into the vise (make sure you aren't gripping the frame in any way). Tighten the vise as much as you can, and then use the frame as a giant lever, and twist the frame around the bench vise. It really does work like a charm. Oh the wonder and glory of simple machines.

I've actually had once instance where that wasn't enough, so I took an air powered heavy duty grinder, and ground the hell out of the protruding portion of the fixed cup. Then, when there was a sufficient notch in the cup, I took a punch and hammer to it, hitting it counterclockwise (right hand threaded cup) until it finally started loosening up.

i hope that helps, good luck.