I have a 1990 5.0 hatch and just installed both Kenny Brown subframes and a crome MAC strut tower brace. The car handles fantastically!
The subframes are a MUST! The STB is necessary only if you're planning on doing ovals with the car and taking tight turns. I don't do either but I installed it anyways as it does add to the front end tightness, and should I decide to put in a supercharger, I'm all set. depending on how much abuse you put on your car, subframes may be all you really need.
However, my advice to you is DO NOT install a STB unless you have subframes cause all that twisting will work it's way up to the sheet metal where the STB is bolted down and turn the drilled circular holes into wide ovals!!!
An STB should be used as an aid to the subframes.
Subframes should be your fstarting point/first priority to stiffening up the chassis before you add any other mods.