I just scored an excellent example of this 1975 model kit made by Aurora. There was another kaiju kit in the series (Rodan) that I am still tracking down. I like how the box art exclaims "This Three headed terror known as Monster Zero, attacks a Japanese air base." Even though that doesn't exactly make perfect grammatical (or punctuational) sense, it's nice to see the Monster Zero name used.
The box is a delightfully 70's purple, which helps Ghidrah to stand out. When this kit was re-issued in 2000 by Polar Lights, they retained the box art, but spruced up the background.
Below are scans of the Instructions. Before we go there, I have to say that their "Painting Instructions" included some bizarre color choices:
Well, okay, Aurora, if you say so. I would love to see what this would have looked like...it would have been very trippy, all pink eyes and orange mouths, like a black light poster of Ghidrah or something.