You popped into a 60-66 club Turbo. Then you something like "Everybody knows rap is better than Zeppelin." or "George Washington was a homo". Your taking a piece of history and tearing it up because "it aint what we want".
If it wasnt what you wanted you could have just as easily bought a chevy like they said. GMCs arent just another truck for so many different reasons. They were actually designed differently than a chevy with different benefits. And they were made in smaller numbers than Chevy so there are less of them out there.
Maybe you didnt know that but give it a couple of days and
maybe read up on the time and work put into GMCs to make them better. Then if you still dont appreciate the prize you have in that 351 then go ahead and tear it up.
If you do appreciate it then maybe dump the whole truck on Ebay and get one of the thousands of chevys there are out there.