Not sure about the fuel delivery conversion, but as far as the exhaust goes, you can use any headers on your car made for a 79-93 Stang with a 5.0L.
The reason they say 86-93 is because that's the years the cars came with dual exaust, not a Y-pipe - in order to run dual exhaust on your 84 you'll need to pick up one of Ford's double-hump tranny crossmembers
or you won't be able to run the exhaust without the passenger side pipe almost touching the ground. You can buy them anywhere - I just linked to Jegs because that's where I got mine from, but looking back, I could've just bought it locally if I'd looked around though.
I bought a set of ceramic coated Hedman E/L shorties for my 83 and they're great IMO. My car's still mostly stock and I'm not making huge power, so longtubes aren't needed at this point.