If you didn't have a problem before you replaced the intake manifold, and you have a problem now, go back to the last change, the intake. I'd replace the intake gaskets/ valley pan, and check the head surfaces on your new intake to be sure they are flat and not gouged or something. Did you reuse the valley pan, or install a new valley pan? Oil fouled plugs would be expected if the intake ports are not sealed off from the valley and is sucking oil into the intake, and low intake vacuum would be expected if the intake isn't sealed to the heads. Both of which would indicate poor gasket seal.
Oil on the headers would tell me your valve cover gaskets are leaking, or the front and/or rear corners of the valley pan are leaking.
You pulled the intake, change the oil after you replace the intake/valley pan gasket, a bit of antifreeze could have gotten into the motor with the intake manifold/gasket change. Change it again in another 500 miles, just in case.
Years ago I bought a rebuilt 440 pretty cheap, it was using a quart of oil every 500 miles. I changed the intake valley pan gasket and solved that problem. It was a great motor. The guy didn't believe me.
