It's funny every so often you'll encounter a 60-66 Chivy that's been converted to a GMC V6, it is rare
My first encounter was at the age of 16 in Auburn California, it was a 64-66 Chivy pulling into an alignment shop, I heard it and wanted to see what was up, the owner popped the hood and it was a 305 V6. My father and I both commented to the owner he had a great engine in his truck, it seemed that he had just bought it.