The fuel pressure regulator is located beneath the upper intake plenum.
Fig. Fuel pressure regulator assembly location (6)-4.3L engine
Either the fuel pressure regulator is bad, or the poppets on the spider fuel injector are leaking. You will know if they are when you pull the upper intake manifold as it the surface of the lower intake will be wet with gas. If this is the case you will need a new spider injector system, which costs about $350. If surface of lower intake is dry then you just need to replace the regulator. Make sure you replace the o-rings as well, which should come with the regulator.