if torque is what you want, a towing camshaft (many companies make cams for the 4.9L (comp cams does, wolverinek, etc.) as well as small diameter headers or extrude honed stock manifolds (your truck has the dual exhaust manifolds each one for 3 ports, right?) with mandrel bent 2 1/4" exhaust with a low restriction muffler and catalytic converter, GEARS (should make a HUGE difference). The induction on those trucks in meant for high velocity, low volume so it should produce good lowend torque with it the way it is, but if you feel like spending money, extrude honing the intake/head or porting them yourself would increase volume and retain a lot of velocity as well. Good luck.
- i've heard that at best just a replacement type air filter like a K&N will give you like 2 or 3 horsepower. Me and a guy at work read the voltage coming from a mass air meter on a 5.4L expedition with a newer K&N compared to a brand new motorcraft filter and the output was almost identical even at part throttle (couldn't really test full throttle, but K&N claims like 20 HORSEPOWER on all vehicles which is very unrealistic).
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT