Greg,
If the engine was originally installed in a CJ3A, it will have a "3J" prefix.
The L134 engine was used in a number of vehicles...starting in the 1930s...and all the way to the mid 60s. I can't say for sure, but in this day and age, I'll bet it's more common to find a non original engine in a Willys Jeep.
This serial number is unusual. It's got me scratching my head. Some of the fonts look correct, but there are different "3s" and different "7s". Also, the "8s" don't look period correct to me.
What is the casting number along the lower edge on the passenger side?
Tim