I don't think those are spacers. Those sound like alignment dowel pins. Go back to the shop that did the work and explain that they were installed when you took the head in and you didn't get them back.
That is correct Jesse. They are the same as a 302/5.0.......Thats what they are, alignment dowels for the cylinder heads.
You can usually get them from a Napa. I think you can get boxes of four from any of the other part stores as well, like Autozone or O'Reilys. Felpro and the other gasket manufactuers should make them.
I could be wrong, but I think they are the same size dowels that the 5.0's and 302/351's use.
My pins stuck in the block not the heads ? lol
Secondly, who milled your heads, ask them what they did with them ?