My book is "Cars and Parts Catalog of Chevy Truck ID Numbers '46-'72 Pick-ups, Suburban and El Camino" and it says that the "J" stands for the year (1951). The "A" stands for regular which means standard 216 cid. and "C" stands for the plant (Flint). Other prefixs for that year were, JAA, JAM, JAC, and JAP all for the 1/2 ton pick-up. They all had the "A" and my book lists nothing for cars so it's possible it could be a car engine but I have it as a 1/2 ton pick-up engine. Doesn't really matter, it's still a 216. I would look for a 54 or newer 235 or convert the 216 to inserts for reliability and durability.
Just one guys opinion,
Scott
