As far as I know there was no stock or GM accesorry clock or tachometer for that vintage of GMC or Chevy. Your best bet is to watch ebay or hit the junk yards/swap meets and look for something that looks the part. Otherwise you could buy a newer aftermarket tach.

I may buy a cheap aftermarket and design a new face for it to match the original gauges and have a friend of mine print out a vinyl decal for me. I think anything else would look out of place.