Don't see too many 91 stock 5.0's these days...but there's nothing wrong with adding some mods to make it kick some *** on the street. There's a lot you can do with those cars from completely overhauling the engine to slapping on an intercooled supercharger. I'd post a couple questions in the 79-95 5.0 section, probably get the best responses there regarding your modification questions.
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.