NICE FIND! ! ! !
Try putting the stem bolt in from the bottom and wiggle the wedge with it. I'd also suggest penetrating oil intstead of rust killer. Hopefully you haven't messed it up already. Rust killer works by converting the rust to a compound that seals and hardens. If it's the same stuff as I used on one of my old trucks you may have just glued the stem in place for all time. But it may be that it didn't penetrate all the way or perhaps YOUR rust killer is a rust remover. They are highly different things.
Anyhow try some penetrating oil and see if that helps. And work the wedge loose from the bottom by using the stem bolt to wiggle it and pry it as best you can.