Hey 5.0 HO:
No I would not sell it unless you want to get the notch because it is what you like or you are just going to go hardcore race which it does not sound like you are.
I would not get a mass air right now either, the mass air won’t do anything for your performance and if you don't change your cam or injectors you should be able to get by without it for now.
I would start with letting it breath a little better by adding heads and or an intake. The most important thing is to match up your parts right or you might not see a big gain.
Find your self a good set of used aluminum heads or go with some gt-40 heads, these work good especially with a heavy SD car.
Next get your self an intake and a 65mm throttle body.
The TFS street heat intake works good but you can probably find a used cobra intake for less that will work just as good for your combo.
Then I would start thinking about the mass air conversion, injectors, and a cam.
You should be able to get your self a good set of heads installed for under $1000 and that will wake up your Mustang.
Hope this helps.
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James Cox
nochevy@hotmail.com
1991 Mustang LX
12.565 @109.38mph 1.764 60ft