Take two cold beers and call Xweb in the morning...

Problem solved... As I worked through the second well-chilled dose of the standard prescription for mechanical frustration, my wife asked whether there was any way that I could mate the end of the new-style cable sheath into the fitting from the end of the old-style cable sheath... like cut some threads or something...

Bingo! Eureka!

The nose at the end of the new cable sheath is 6mm and change, just about right for a 1/4-28 die. The hole in the back of the fitting for the old sheath is just a skosh less, and perfect for a 1/4-28 tap. So I threaded the nose at the end of the new cable sheath, threaded the fitting from the old cable sheath, screwed the old fitting onto the nose of the new one... And there I had a new-style cable sheath with the old style nose that will fit the narrower cable tube on the '74.

It's all installed and on the car now.