The driver side O2 sensor wire is anchored to the rear plenum and it’s hard to see. Make sure it’s freed otherwise the plenum won’t come off.
When putting the plenum back on - a couple pieces of rod can be placed in two of the engine block’s threaded holes to act as a guild when setting the plenum in place. It also helps to keep the gasket aligned. Helpful to have some handling assistance with the plenum since it can be awkward to handle alone...