XFactor...
I would recommend pulling the plenum all of the way off to do it right. My reason is this - it's a PITA to slide the plate in there with the gasket on top AND on the bottom (need to use two gaskets with it).
All the time I spent holding up the plenum and trying to slide the plate in, I could have had the whole thing off. Also, while you have it off, you can clean it up and such, as the back side usually gets kinda dirty.
The only screwup I had was that I stripped that one mother of a bolt on the passenger side, but most cars are running around with the same stripped bolt (I think they stripped em at the factory).
Just be careful that you don't kink the vac lines when removing that pass side bolt, take your time, and you're home free.
Rich