I had the same problem and bought different injectors that did fit. try taking the oring out and seeing if it still doesn't fit, the problem with mine was the plastic clip under the oring was to big and wouldn't allow the injector to go in. FIC said it was fine to just take the plastic ring off cause of the way the injector hole in the intake was but I decided to just get new injectors instead.
also make sure you are running the right oring, should be orange I believe
also the tune would need to be adjusted with different injectors

 

yep ls2 injectors and plug adapter needed for mine too. then it fit perfect. watch those plug adapters though, the pins can be pushed in too far and won't run right.