Everyone has a "secret sauce" so here is mine. ATF(the GM stuff, I don't know the Ford stuff) mixed with acetone. 50/50. Let it sit and tappy tap tap. Repeat and repeat. What you probably have is some rust bonding in the bore. Pull the oil pan off and turn the engine to the cam heel for that lifter. That will give the biggest gap between lifter and cam(just a safety tip). Squirt some up into the lifter bore if you can. No abrasives, and no rush. It can take days. Squirt, soak, and tappy tap tap. Cool beer seems to help.
