convert the car to mass air before you do anything else speed density can only compensate for a 15-20% in crease in airflow if it hasn't started yet your going to get that wonderfull surging idle shit S/D mustangs are fond of.....
you can buy a complete kit from ford or piece one together
you need maf sensor, intake elbow, conversion harness (comes in the kit or availiable seperatley through painless wiring), and a mass air computer A9L or A9P are the ones to look for, the ford kit comes with an A9S (calif emmissions to make it 50 state legal) its ok but the federal computers are a little more aggrsssive
I have a ford conversion harness it plugs directly into the back of the computer to repin it for mass air and adds the extra wiring to the maf....
the biggest bang for your buck on any 5.0 powered car are cylinder heads the stock heads are going to hold you back big time
your going to add 20-40 hp with all the bolt on stuff you toss a set of heads in the mix and now your looking at a 60-100 hp gain depending on the head you use
Good Luck