The injector o-rings need to be bought separately. The kits come with similar looking, round o-rings but I remember none of those fit, like you said. I bought the ones for my injectors at Advanced. They were also Felpro. Don't have PN for ya, but they should show up in their computer systems once you tell them it's a 3800 engine.

Buy extra ones, they like to tear when you're pushing in the injectors.