Port matching alone i've heard is worth about 5-10 horsepower depending on how much the ports are misaligned (if much at all). You probably won't notice any change in gas mileage although you may feel the difference through the pedal. If you're going to port match, you also might as well clean up the ports a bit and use sandpaper rolls to even things out. Also getting rid of the thermactor exhaust bumps in the ports (you can completely get rid of them and not have any drivability problems from what i've heard) will give you from what i've heard like close to 40% more flow through the exhaust ports if done right. On the exhaust side, i've heard of people polishing it to a mirror finish for maximum flow. i've also heard of people leaving a satin finish with the sandpaper so carbon sticks to it, thus retaining heat and energy within the exhaust pulses so that they leave faster, but i don't know what way is better. Anyways, good luck, light porting and cleaning up will go a surprisingly long way.
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'84 Mustang 5.0 LX
My car