Both are good upgrades. You might want to wait to do the headers after upgrading your intake manifold and mufflers first. Underdrive pullies are good but if you have a fox body mustang I would not install the alternator pulley if you have the stock alternator. Those are extremely low output to begin with and an underdrive pulley on there will make it even worse.
I'd change the crankshaft and waterpump for sure though.
Headers in stock shorty form come in equal length and unequal length (factory style). Equal length do provide more power and torque but are much more time consuming to install and work on down the road. I've used both and I just hate working on equal length so I stuck with unequal.
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