1966 Barracuda
At the top of this page is a VIN decoder link. With the 64 date I'm guessing the engine is not original.
That may not necessarily be true.
The Block was cast on 12-20-64. this is the middle of the 65 model year (they usually start the new models the last two weeks of June and first two weeks of July). They stagger launching all the assembly plants as some engineers may have more responsibility than just one vehicle. They may need to be present at the launch to make sure that the new vehicles are assembled as intended. For the engine guy may have his engine that he works on in more than one vehicle, this way he can go to each plant that his engine is launched in to verify that the launch is being built as intended/designed.
The block may not have been machined properly the first time and may have needed to be repaired, lets call this a "repair block". when I used to support the block machining line, we would sometimes get some oversize lifter bores. We would contain the blocks with the oversize bore and mark which bores were "out of spec" with a paint marker. When we would accumulate enough of these, there was a local machine shop that we had already certified to repair the lifter bores. They would sleeve the oversize lifter bore on and then remachine the bore and oil passages for us, and then return the blocks to be made into engines. These repair blocks are now within spec's and are good for using.
They would then build these blocks into engines. It could easily take a few months for the blocks be sent out and repaired. Thus, it is possible to have a block that is cast 6 months to a year before the car was built. The best way to tell if the engine is original for the car is to check the VIN number on the block to the VIN on the dash, but back then I don't think that they put the VIN on the blocks yet.