I've got a 92 rag that wants tofloat at speeds over 85. I know lowering it is the surest way but I'd rather not if a front curtain or a stiffer sway bar will get it done.
You may want to check the tie rod ends to make sure everything is in good working condition before modifying. Certainly, it will feel more unstable at speed with worn out tie rod ends, and those can go out frequently.
Otherwise, as stinger said. Things to make the steering more precise, like a stiffer sway bar, stiffer springs or bigger wheels