68 340 engine or good block wanted

Thats not hard to believe. The block date is and should be before the car build date.

6-23-67 to 5-31-68 is a wide gap but as long as its not the other way around Im good with it.

The build date of the car may be much closer to the engine than the date he got the car.
That could be true. He test drove one 3 days earlier, but wanted a red one. That's when they told him that they had one coming in on Friday as a dealer stock car. He bought it the same day it came in (Friday). I don't know how long they stayed at the factory before they were shipped to the dealer.

I just checked the build sheet, but it's in such bad shape, I can't see if there's a date on it or not. Up at the top it has the sequence number, then down the page the different options. The top left is missing, don't know if there is even supposed to be a date on this.