[awhtx]

5H25 is August 25, 1965. So if the block was cast on August 25 and it takes 2 weeks to a month before it becomes a complete engine then it would be correct for a car assembled in the Sep/Oct time frame. Ford was cranking out Mustangs as fast as they could assemble them during this time period so I would say that the casting date and assembly date would be within no more than 2 months of each other. Your block would be correct for an early '66 Mustang.

 

[awhtx]

Yes, just forward of the driver side head. If the deck of the block has ever been machined it may be gone.

 

[awhtx]

Assembly date is 5H26T. Data plate says build date of Sept. 9.

I don't see how the block could have been cast one day, all the machining done, and then assembled into a complete engine the next day. But when you're cranking them out as fast as possible I guess anything can happen.