My August 69 Ford Parts Book update lists this washer for the 289 spec. as part # C3OZ 6378-A. It shows the dimensions right in the book as 1 3/4" OD, -21/32" ID, - 1/8" thick.
For all other applications the standard washer is part # B8A 6378-A. These dimensions are 1.76" OD, -.77" ID, .212" Thick. This is correct--The standard washer is thicker.
This topic has been bothering me so I crawled under my K-Code and removed the bolt to double check the washer. It measured exactly with the parts book dimensions. Now I'll go back and torque the bolt and washer back in. Hope this helps out my fellow Mustanger.