Measuring the Headrest
When I lived back in Kansas before I went to work for Layson's, I recovered the seats in my 68 Satellite, 70 Charger, and 70 RoadRunner(wished I had them all back now......<sniff-sniff>).
I bought my hog ring pliers at a "Farm & Home" store. I use the straight type of pliers as thery are a little easier to get the listing wires ringed from the backside of the seat bun. Also, get the spring loaded kind as they hold the hog ring in the jaws while you get it in position to clamp it shut.
If the handles are bare metal like most of them are, you might consider dipping them in some tool handle dip. After a couple of hours of hog-ringing, your hands will appreciate it.