I have an 1988 mustang motor with a 303 cam and just bought it about three weeks ago and its misfiring i have it running open headers and i everytime i pull my plugs 2 of them have oil deposits on them what could be the problem? Could it be a valve burned due to lack of back pressure

 

Back pressure has nothing to do with your situation.
Get a leak-down tester and do the test, best way to find out whats wrong with an engine short of tearing it down.
Harbor frieght has them for about $50-60...........

 

Worst case scenario what could it be, im thinking maybe rings an if its rings then ima go ahead get it bored. 30 over and get a rebuild kit use my 303 cam and my crank that i already have