That casting number was used from 64-66. The casting date will be on the other side of the block.
If it was original to a 64 Barracuda, it's not the right date for me.
I had a really large batch once. I put them in a cement mixer with 50 lbs of sand some water and some purple degreaser. It got most of it off, but I still had to media blast them and chase the threads before sending them off for plating.