It could have been a NOS engine that never was installed by a vehicle manufacturer.
I could not find the page for a 1960 but here is one for a 1959 Chevy truck. The serial number can be used to identify what type/size truck it was installed in and what options (RPO) was used. At one time and in some states, the engine number may be used in the title. In your case, the engine is a transplant so I would think it's not a real concern.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
