Do a complete tune up of what you have - spark plugs, cap, rotor, spark plug wires, fuel and air filter. Dump the codes and fix any that come up.
Avoid cold air kits - they work good on the 2005 and later Mustangs but don't do much for the Fox body cars. Above all, stay away from setups that put the uncovered air filter in the engine compartment: they suck up hot, turbulent engine air and cost HP rather than improve it.
The stock airbox minus the silencer horn in the fenderwell is one of the best you can have. A
K&N flat panel filter is a good choice as long as you don't use too much oil to coat it with.
Look at a 65 MM throttle body as a possible next step. Don't get foolish and try a 70 MM throttle body, since it is overkill for what you have. The V8 Explorers 1995-2000 have one that can be modified to fit if you are on a tight budget and have some fabricating or machinist skills.
If you want to stay with the 19 LB injectors, consider a larger MAF. The OEM Ford MAFs are not calibrated to injector size. The injector sizing is done in the computer’s internal programming. Aftermarket MAF’s “lie” to the computer in an attempt to fool the computer into changing the injector pulse on timing to match the increased airflow.
The only other MAF that is a one for one swap for a 93 and earlier 5.0 Mustang is the 94-95 Mustang 70 MM MAF.
94-95 Mustang GT MAF - $40-$100. It is 70 MM instead of the stock 55 MM on regular stangs built prior to 94. It uses a slip on duct on the side that goes to the throttle body and a 4 bolt flange on the other. You need a flange adapter to fit the stock slip on air ducting that goes to the air box. Wiring plugs right in with no changes. *1 *2
Once your replacement 70MM MAF is in place, disconnect the battery for about 10 minutes. When you reconnect the battery and start the engine, the computer will relearn the settings for the new MAF.
*1.) Metal flange adapter
http://www.kustomz.com/cat3.html Buy the TR70 for $44.95. Or spend some time on eBay looking for one that may fit.
*2.) MAF & sensor interchange
The 94-95 Mustang 5.0 MAF & sensor is also found on:
1995-94 Mustang 3.8L F2VF-12B579-A2A,
1994-92 Crown Victoria 4.6L F2VF-12B579-A2A,
1995-94 Mustang, Mustang Cobra 5.0L F2VF-12B579-A2A,
1994-92 Town Car 4.6L F2VF-12B579-A2A,
1994-92 Grand Marquis 4.6L F2VF-12B579-A2A,
Evidently the –A1A, -A2A, AA, etc. on the end of the part number is a minor variant that did not change the operating specs. You should be able to ignore it and have everything work good.
A 155 LPH fuel pump, 24 lb injectors and a custom dyno tune & chip would be the next step after doing the tuneup addtional items listed above and fixing any codes you find. You may have to replace the MAF as part of the process.