Flooding Chev Vortec V6 . Fuel pressure regulator

Check the fuel pressure regulator. Pull the vacuum hose off it, If it smells of gas, Replace the regulator. Diaphragm broke and is flooding the engine threw that vacuum line. You'll also need to replace the spark plugs. Gas will foul them out also.

Well if it is flooding, either something is shorting out the injectors and causing them to be wide open, or one injector is stuck open, or you have a pressure regulator leaking liquid fuel into the intake.
Are all of the spark plugs wet ?
Can you start the engine if you disable the injectors ?

Who is doing the work on or testing of your vehicle?

What has been done so far,how & the test results?

You first need to run a fault code test on this engine, to do that go here.

The fact the fuel pressure does not hold after the pump stops is a big problem indicating a fuel leak into the upper manifold, which would cause the flooding. This leak could have several causes, but since the problem is still there after replacing the regulator, it sounds like the problem is with the injector in the spider assembly.
It sounds like you have a design 1 spider assembly, with only one fuel injector distributing fuel to the six poppets. If that is the case, I suggest you try replacing the entire assembly (which will include a new regulator) with the Delco design 2 version which has injectors on each poppet.

If it acts like it is flooding while idling, but it runs fine on the road, it could be running too rich and flooding it out due to leaking injectors, bad fuel pressure regulator, or excessive fuel pressure.

I would start by replacing the Fuel Pressure Regulator since it doesn't cost as much as the injectors.
Check your fuel pressure using a Fuel Pressure Check Gauge to see if the fuel pressure regulator is maintaining the proper pressure.

Vacuum is attached to the Fuel Pressure Regulator to control system pressure. When the fuel pressure regulator is not functioning properly it causes the fuel pressure to be above normal operating pressure. This condition causes excess fuel to be injected causing a rich mixture that produces black smoke.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

another issue that causes flooding in the Vortec 350 ( which, I assume you have) is the fuel injection spider. The spider assembly also contains a fuel pressure regulator if faulty can allow too much fuel to pass through the injectors fouling the plugs (a common problem), While your in the upper intake you might as well take care of your lower. A leaky inner seal on the lower intake can cause out to ceep in to your heads causing a faulty spark.

fuel pressure regulator.

the probem is probably in the injector assembly whichis under the intake plenum.after removing it you will se the plastic tubes and ijector popit valves unlike actual individual injectors.this whole contraption must be replaced as an assembly.you can get repair manuals from this site or go to you library.it may actually be flooding as these had problems with leaking fuel pressure regulators which i think are also only part of the whole assembly.

Just a guess but if you don't have an igniton problem, it may be a bad fuel pressure regulator. If the regulator diaphram fails, the engine will pull raw fuel through the vacuum line that goes to it. Also if it fails to bypass, excessive fuel pressure will flood the engine. Check the rail pressure to spec.