If you are not dealing with a loose or tight headset, you are dealing with geometry. One bike needs a 90mm stem and the other 120mm, so you definately have geometry differances. You probably won't get much of a handling change with the stem. I would think that a fork change is the only thing that will do it.

I think you are trying to make a '80 Corvette handle like an '03 Porsche. They are just too different to get the handling the same.

PS. Before anybody gets upset, you can interchange Corvette and Porsche in the above statement. They just handle WAY differently.