Here's my opinion:
'89-'92 LX 5.0L notchback (not the hatchback)
Here's why:
Speed density fuel metering system adapts better to modifications ('89+ 5.0L's all have speed density, which measures the incoming air rather than calculating it)
Notchback is a bit lighter than the hatchback and a hell of a lot stiffer (which is the fox mustang's weakness anyways)
Better brakes than the pre-'87 (not sure on when mustangs first came with 10" rotors rather than 9") and also 10" drums on the stronger 8.8" rear axle.
stock these cars could easily run low 14's and were the car of choice for the police package because of the lighter weight and frame rigidity.
Good luck on your purchase!
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'84 Mustang 5.0 LX
My car