While working at a Chevy dealership some guys would come in and order a complete engine assembly. If it was the current year, if you wanted to wait, you could order what was known as an assembly line engine. This had from air cleaner to exh manifolds. What you put it in was your business. This gave the appearance of a “factory built” car.
One time we ordered a 61 283 Fi engine assembly. Then the dealership installed it in an Austin Healy 3000.
Very possible the Chevelle got this same treatment.
Who knows.