I have a 96 1/2 ton 4X4 with a vortec 350. I recently replaced the intake gaskets (also cleaned inside the plenum) Sence then it has developed an intermitent misfire(p0300). I have replaced the cap and rotor. The plug wires and the plugs. When that did not change anything I changed the fuel filter and checked the fuel pressure. It checked at 58 lbs. What am I missing?

Fuel pressure should be 60 to 66 PSI. Anything below 60 PSI can set a DTC according to the C/K 96 GM Service Manual.