I was at a local car show and a guy had a '60 GMC there with a 90's S10 Blazer tank under the back. He said it was an aftermarket kit that he got.
I had previously mounted a 90's S10 Blazer in the back of my '61, and was surprised to see others using the same tank.
Bottom of
THIS PAGE I show the tank mounting
VIDEO I did featuring the fitment.