I went through everything mentioned in this post, including your syptoms trying to locate my
oil consumption problem in my first 418W motor. Replacing the intake manifold gasket helped since it had been installed incorrectly by the original engine builder at House of Cobras in Anaheim. Went from using a quart every 500 miles or less, to a quart every 600-800 miles. Finally had enough with
oil smoke out the pipes and rebuilt the engine after 3 years and 11k miles. During tear down, the rings and piston ring lands were shot and not installed correctly during assembly was the major contributor as was the intake gasket mentioned. Best of luck with your motor Wayne.