It's a little murky for model year 1966 because that was during the period of time when Pontiac didn't stamp the VIN on the block.
Full VIN's were stamped on the block on '63 and EARLIER engines. From '64 thru '67 no VIN, full or partial, was stamped on the block. Fed law mandated the partial VIN stamp from '68 up.
The June date code is also a little problematic because generally, the engine foundry "switched over" at mid year and started casing engines for the next model year about that time. Look on the side of the block and count how many of the large cating plugs ("freeze plugs") are on each side. If there are only two, then it's a 66 or earlier block. If there are three, then it's a 67 or later block.
So.... since you have a 1966 --- if you've got a block that has a set of "believable" date codes and head codes, and appears to be the right model year, then personally I'd have no problems with claiming it was the 'born with' engine just because of the absence of evidence to the contrary. You do the best you can do with what you have, and also do the best you can to be honest, then go from there.
If someone were to try to "pin me down" on it, then I'd take the time to explain that in 1966, it's not possible to "prove" one way or the other from the available information and leave it at that.