I was wondering do I need to replace oil leak or do I have bad oil pump?

Hello,

I have a 72 Dodge Dart 318 that have a oil leak. It looks to be coming from the top back of the motor because I see a run down the engine falling on headers, and there is wet oil on the top back of the intake manifold. So my question is could this be an header problem or could oil be coming from intake manifold. Also when I check oil it shows full even though I know I have lost oil. When I start car it is at about 50psi (oil), and as it run it drops lower and lower until the car basically wants to shut off. If Im driving car is fine, but when I pull up to light and it idles the car wants to shut off. If I continue to give gas the car would continue to run. Also if I put the car in park the pressure goes up, and the car continue to run. Although I know I need to get leak fix. Could this be an oil pump issue? Or something else.

Thank you,
Marco