Yah the stock leather seats look alright, but really don't have much support and aren't very comfortable after a long drive if they're pretty worn in.
Fortunately for you, you have black leather/interior. You can probably find Cobra/Mach 1/Bullit seats out there...from either a wrecked car or a private seller whose upgraded to racing seats.
There are some fairly mild Corbeaus for a reasonable price...these in black wouldn't look too bad.
MUSTANG CORBEAU SEAT RECLINING GTS II PAIR
12 Black GT500 SVT PP l 2.6 Pulley/Idler l Airaid l o/r X l 305 NT555R's l Recaros l
04 Azure Blue Mach 1 / 03 Sonic Blue 3.8 / 14 Sterling Grey F150 FX4 402A 5.0 / 11 GT500 R.I.P.