General LSX Automobile Discussion - low power during acceleration

got my 2002 5.3 truck motor going in my 86 K5 blazer recently. Idles and coast good but not much power during acceleration and is setting lean code on both banks. Fuel pressure is about 55 psi when idling and increases some when I raise rpms while in Park. Sometimes I have seen as low as 50 on the gauge at idle.

I’ve looked at all my fuel line connections and do not see any leaks. New ac delco filter. One thing I’ve noticed is the fuel pressure leaking down pretty fast after shutting the motor down. Seemed like before, the gauge would stay above 50 for 2 or 3 hours. Would this indicate a leaky fuel rail regulator or injectors and this be the reason for low power.

I have not pulled plugs yet. Might could isolate it to one or two cylinders.

Is there a particular port on the truck intake that would give good readings on a vacumm gage.

Thanks