Blazer misfire and low fuel pressure

Hey guys I have a 01 blazer with 4.3 v6 in it 2 door. The vehicle was running rough so I did a tune up to it, spark plugs cap rotor and wire. I get a p300 random misfire code and the car start hard. I put a fuel pressure gauge on it and when cranking and starting I am only getting 55-58 psi fuel pressure and then when the car shuts off within 30 seconds it drops to zero. It doesnt immediately drop to zero but over 30 seconds slowly drops which I know isnt good and causing my hard starting. My misfire and rough idle could maybe be from not enough fuel pressure? Since this car has all metal and hard plastic return lines I cant easily close the return to check to see if the pressure increases. Could the spider injector cause this. Having trouble trying to figure if its the pump, fuel pressure regulator or spider injector. I dont have a gas smell in the oil. Any help would be greatly appreciated. Thanks