Big blocks use a push rod between the crankshaft & pump, the aftermarket push rods are infamous for wearing down & becoming too short.
I'd pull it & measure it.
Pressure isn't likely to come back if this were the case
It could if the demand were low enough as with the shorter strokes it could produce the pressure until the demand ( flow) increases. I'd checkk the rod, if in spec, replace the pump
Sounds like all the measurements are at idle, parked. Not in use going down the road?
From the OP's 1st post
It started running poorly and I noticed it had 3 psi fuel pressure. I blew out the line to the tank with air and made sure the fuel filter was not plugged, fired it back up and had 9 psi.
Then I took a 0.67 mile, 25 mph trip around the block and had 7.5 psi when I got back.
Then I took a 2 mile, up to 55 mph trip around the bigger block, and when I got back I had 6 psi that dropped pretty quickly back down 3 psi as it idled, basically where I started. Not even enough pressure to fill the fuel bowls
So IMO, possibly a rod or pump
