Colin,
I had some install-yourself seat covers made up for my "71 142 by Custom Interiors, P.O. Box 51174, Indian Orchard, MA 01151, Phone: 1-800-423-6053. They have a web site, but I can't find the URL right now. Cost me $216.95, including the shipping. Mine was a pretty basic set, but they do fancier stuff too, and they have patterns for just about any old car you can name. My seats were in pretty sad condition, so I had to do quite a bit of sewing and stuffing before I put the covers on, and rather than just attaching them with hog rings and tie-straps, I did some extra sewing to get them nice and tight. Came out fine - maybe not exactly like the originals, but certainly with the "period" look. And the price was right.
Bob S.