I guess it could be. Mixing parts from different years?, who knows if it's all the original parts??
As an aside topic. A person I know, cannot call him a friend or acquaintance.. assembled a 66 fastback Mustang. It was a car I put together as part of a deal and it went south badly. In any event.. he had a SBF that he was unsure of the year and he didn't know how to tell. And he had bought a late model ( late 80's ) 5.0 that he wanted to put into the Mustang so he assembled all the parts that he had and got the car running. It looked really nice but the motor shook something awful! He wasn't aware that early SBF used a 28 oz flywheel weight and Later 5 point ohs used 50 oz... He mixed the 5.0 with his early 302 flywheel! I would only smirk... told him once he should find and fix the problem.. Another guy who knew my history with the car said he was trying to buy it, I asked him if he tried to drive it. He said no but he heard it run. So I told him what the problem was and swore him to secrecy and to use that knowledge to get a better price. Which he did and then brought the car to me and I swapped in a new flexplate and then drove it back to the previous owner. Revved the hell out of it with a nice burnout! Something they could never do! Satisfaction guaranteed!