I found that on my 2.3, the stock alternator was JUST adequate. IE meaning with the A/C on full, heater fan blowing on high, cooling fan running, headlights on high, and cruising 75 mph, the voltage at the battery was 12.9. Hit the brakes (brake lights) and it would drop to 12.7-12.8 which was exactly the voltage with the engine not running at all.
Swapped in a F-150 3G style alternator (not sure of the amperage) and voltage at IDLE (800 RPM) with EVERYTHING on (A/C, HVAC fan on high, tail lights, high beams, brake lights, radio on, interior lights on, etc) is 13.6V. Doesn't help MPG or HP, but it's quite a bit more confidence inspiring especially since I drive in the dark about 75% of the time....and of that 75%, it's almost ALL between midnight and 4am. Not a good time to have a alternator fail!