removed the eight runner to plenum bolts. But the plenum is not budging, no movement at all. What's going on? Thanks for the help.

 

91,

You may have to loosen at least one set of runners by loosening the lower bolts. There are only eight bolts holding the plenum to the runners, so you've found them all. There is a good chance that the gaskets are sealed/bonded to the plenum, and that it is somewhat "pinched" by the runners.