It would be a hellava coincidence that someone put in an engine that happens to be dated exactly when the original motor could have been dated. An actual GM document says they used 261. Even if a PO blew the original motor and found a correct one, it's correct! "Numbers matching".
I think I would advertise and sell it like that without any worries. As Tony indicates , most early, mass produced vehicles do not have paper work. I believe they can be called matching numbered if the cast codes are correct. That's all there is. Certain era trucks are viewed to be orig without paper. To be frank, it really does not matter for that truck that much, except to the owner. The price is not affected on that truck. The value points are 60-66 1/2T, Short bed, Fleet side, 4x4, engine, trans, big window, Deluxe, condition. 1st year for coil (2wd), last year for wrap windshield. Original engine and such would mean one or two more buyers. 454 w/400T, a few more different buyers, etc.
