I have a 71 mach 1. The original 351 engine is no longer there, and now there is what I believe to be a 429 or 460 in the car. After a little detective work, I learned that the block partial vin is behind the driver's side of the block below the heads. With a little help from some silly putty (which I myself think was an excellent idea.) I extracted the number 7Y928486 What my problem is that I can't find anything like that. I think the assembly plant was Wixom MI, but I'm not sure. Any help would be appreciated.
As you know, that number is a partial vin match for the vehicle.
On the 351C the block number is located above the starter. I am not sure on the 429/460 but that is the number you will need to more accurately identify your motor.
Someone with big block knowledge will jump on and give us a hand.
There are normally 2 sets of #s on a Ford engine block of this era. Behind the starter & above the pan rail is a 'casting' (cast in at foundry) number which can reveal yr/design/revision of the block. The yr could be approximate 'cause it more accurately indicates yr of design, not necessarily yr of manufacture.
The other number (behind cyl head) is 'stamped' w/ a die when engine was installed. It indicates yr of install, VIN code of engine & consecutive s/n of vehicle originally installed into. When this stamped # matches the last sequence of numbers on the car VIN, one can claim the engine is 'matching #s'.
Both cast & stamped #s help ID what is there, especially if they 'match'. Your "7Y928486" appears to indicate a '67 (probably) Ford/Mercury vehicle w/ a 390 2V engine of 265HP. More can be ascertained w/ the cast # of block. Hope this helps.