You're going to be pretty limited by your budget and goal of maintaining the factory source unit. I would suggest that you go down to the store and try out a few speakers to see what you like best. If your factory speakers are functioning in reasonable condition today you probably won't notice much difference at all except the rear possibly being louder due to more efficient, or lower resistance, speakers. That's pretty much the opposite of what you want since humans are designed to listen from the front.
The single biggest thing you can do to improve the sound quality for as little money as possible is change the source unit out. Even the lowest cost dirt simple Pioneer will be a HUGE improvement over a stock '90s Ford unit.
Whatever you do please buy the wire harness adapters so you don't cut up the factory wiring. You can always go back easily later that way and it's only original once.
'91 GT 5 Speed