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This came in as hard start after sitting.

Checked fuel pressure, was a little low + as soon as shut-off, pressure drops like a rock. Plugged return line + all seemed good. So replaced fuel pump regulator then no change.

Any ideas?

although it really doesn't fit the issue here i've seen mass air flow issues that caused hard starting issues. another thing i'll throw out here. did this all happen after the tank was filled the last time?

Again I am late in posting, so this is for future reference of this problem. If fuel pressure drops on GM truck immediately after the pump finishes it's prime, or the key is turned off, then there is problem. GM trucks (and cars for that matter) are supposed to maintain fuel pressure for quite some time after the pump turns off. This WILL cause extended cranking and hard starts. You say you replaced the fuel pump regulator? Were you referring to the pressure regulator? If so, then you might need to look at the tank unit. I have replaced at least a hundred tank units for the exact same problem. Just my $.02 worth.

does it have alot of black smoke when it starts or runns bad. Could have a stuck injector