new engine getting only 3 psi fuel pressure

[1972BlueC20]

Just got my truck back from getting a new engine installed part # 19210008.

It has a Holley 670 vacuum secondary carb and a Holley mechanical fuel pump.

When I first crank the truck it has between 6-7 psi of fuel pressure (gauge is on dual feed line near carb). After the truck idles for a while the pressure drops to between 3-4 psi. I can also see bubbles start to come up in the fuel filter after the pressure drops.

I cant see any leaks in the lines (rubber 3/8 line from tank to pump and from pump to filter and carb)

I thought maybe my tank needed a vented cap. Currently it doesn't have a vented cap. My truck is a c20 non eec tank. It has a return line for the original style pump with 3 ports. This new Holley pump has only 1 inlet, so I capped the return tube at the tank itself. After the truck idled and once the pressure dropped I took the gas cap off and the pressure stayed the same so I don't think it's the cap that's causing the issue. Also the sending unit and sock is new in the tank.

I think the pressure should be staying at between 6-7 psi at all times correct?

Anyone have any idea why I'm getting only 3 psi after idling for 10-15 minutes??