If it's truly a 1975 frameset, then why not put a 1st-generation Suntour Cyclone derailleur on it ?? Wouldn't the Jack Taylor come with state of the art equipment on it? The cyclone was released in 1975. The 1st sub-generation derailleur can be identified by a set of "shoulders" connecting the upper cage to the pivot bolt. In other words, 2 creases run from the upper part of parallelogram to the pivot bolt. A few years later they got rid of the shoulders / creases and just had a smooth transition from upper cage to pivot bolt. Email me for pix.
With a 28-54 gear, however, I have a feeling that NO 1975 era derailleur will work. Campy and Suntour Cyclone had just about the widest range(s) back then.
Have you considered finding or making a (? my name ?) front "derailleur extender" and use the campy (or cyclone) parts? A derailleur extender was a little U-shaped appendage that replaced the bolt holding the cage together, bolting onto the back plate, and holding onto the front with a zig-zag pin that went through the bolt hole.
- Don Gillies
San Diego, CA