Go to this website, I am a user and fellow foxbody 5.0 owner.
78mm is way to big unless you have some serious mods. I have an edlebrock 70mm and am in the lower 300 hp range and its plenty. You can infact go to big. I dont know if you are a 5.0 or 4.6 ect. but 78mm is big. If you were supercharged and had atleast 30LB injectors and heads, cam, ect then yeah. But if you are mostly stock, dont do it, you wont benifit from it and can actually loose hp