Really strange, took out a 401 and put in a 300??? I wonder what happened to the original block? Rust issues? There was a nice "rust free" 65 hardtop Skylark near me that failed to get any bids on Ebay and he was starting at $2500 with a buy it now of $3500. Too much money for that car.
I too would want to verify that it was an original GS.