Considering what has been replaced up to this point …
I would start by attempting to isolate where the pressure is going.
I would first test the output of the fuel pump with a gauge as close to the pump as possible to ensure your replacement pump can actually deliver the correct PSI.
From there work my way forward towards the engine, IE. just before the fuel filter them after it. Then before the fuel pressure regulator and then after.
At some point, if the fuel pump is producing the correct output pressure, you’ll see the drop off.
It could be a faulty component, or perhaps an internal blockage.
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- 95 Buick Lesabre GM3800 Version 1 Engine – Low Fuel Pressure