2001 SCPI Problems

OK guys here is the layout. This blazer has a little bit of a rough idle, but if you are sitting in park and you give it gas it will not run over 2500rpms. It will knock and sputter like you wouldn't believe. I pulled the codes it is throwing and they are P0300 which is multiple misfires detected. When I pulled the plenum off the back portion of the intake was clean. This lead me to believe it was the infamous fuel pressure regulator. I replaced it. Of course after putting it all back together and upon testing it, the SAME PROBLEM. I am thinking the next step is to test the fuel pressure. I really just wanted to ask if the pressure is low, what else would cause this issue? I believe before I was getting a rich code also. Is there a way to test each injector on this? Could one of the poppets be stuck open? I tried to crank it and 4 of the 6 sprayed fine, but the other 2 I wasn't able to test. I need some help! Let me know what you guys think.