I am currently rebuilding the hubs on my '73, just like the ones you have. These external mount hubs are no longer made by the factory, and parts are hard to find. I searched far and wide for the plastic knobs. You can find them used or NOS around the 'net and on ebay, but they are VERY expensive. For some time, someone was making some dials out of metal. I finally found what I was looking for here:
http://www.parts123.com/parts123/yb....~~~0000024X~F#
$12 is a great price but they will not have the "Spicer" script. They will in fact have the "Ford" script, which I find kinda cool.
This is what you need BUT it is only the knob without the 'O' ring or the pin. Your other option would be to replace them with Warn (very expensive), Superwinch, or Mile Marker replacements. The only reason I did not want to go that route is because I will be running the original hubcaps on mine and the aftermarket hubs would look a little goofy with the hubcap.
But if you are not partial to these factory hubs, get some readily available aftermarket hubs. I'll see if I can find the link to the metal ones.
M