Speed density EEC-IV computers run on a set program table and do not have the capability to adjust to major changes to air/fuel mixture and camshaft changes. Heads, intake, and injectors can be changed as long as the combination of parts matched to each other. You will have major idle and/or driveability issues using almost any larger-than-stock camshaft. The B-303 cam *can* work because the lift & duration are the same on both exhaust & intake sides and the lift is not too far over the stock .444 lift. The issue keeping almost everyone else with speed density from swpping in a cam is vacuum. You would have to figure out how to get enough vacuum to help maintain driveability and usage of your power brakes. Comp Cams makes speed density cams, but they are more suited for use in trucks. Having read your modifications in your signature a cam swap of any kind is a total waste of time unless you just want to do it for "that sound." You say you're going to get a lower mileage and newer Mustang, but take into account the amount of work you will put into it will be no different than what you could do with a 1987 Mustang. Mileage doesn't matter on these cars as long as the body is straight and prepped for add-ons (bracing). Leave the clean and "undriven" Mustang to the purists that intend to keep them forever or for future sale value and modify the hell out of what you have now.