MM, Ooops! I knew that!
Bruter, based on the serial number of your block we can verify that it was put in a car assembled at the Dearborn assembly plant, and it was installed in a Ford. Ford serial numbers started with 100001 and up to 499999 and Mercury used 500001 and up. Of couse, Ford didn't build 400000 cars at Dearborn, but they built over 100000 as evidenced by your serial number starting with a 2.
OK, with that out of the way, we also know that Dearborn was the assembly plant for Mustangs and Cougars and not anything else that I know of. That means that your block was originally installed in a Mustang. Here's where I'm getting confused. The engine serial is either a 66 or 68, but the build date of the engine puts it early June 1967. I'm not sure when they make the model year changeover back then, but I have seen cars built until late July (my buddy's 73 Mach has a build date of July 21, 1973) Did they have that big of a stockpile of engines already built that it could have been made a month and a half ahead of the car build date? No, that can't be the case since your serial number starts with a 2 indicating it was a late-built car, not an early one. Hmmmmm, curious-er and curious-er. A late 68 with a late 67 built engine? Ok, what was the deal with the leftover 289s from 1967 being used in 68. Were they used in early-built cars or late-built cars? I'm not up on the earlier generation Mustangs and Cougars or small blocks either for that matter... Even a Senior Motorhead can still learn a few things!