Cuz the 302 is what Ford calls thier "5.0" But if you want to get technical and do the actual conversion, the 302 is slightly smaller than 5.0. A GM 305 is a true 5.0. All of those mustangs used a 302 5.0 till the new 4.6. As for the high output 5.0, it is a 302 with forged pistons, roller cam, better valve springs and rockers etc it goes on. It had freeer flowing heads and all too. And this engine started in '85 and went to '93 I dont know alot about it though cuz I dont bother reading up on Fords. Here if you want a sire that'll help with engine years and mods go here.
I actually found that site when I typed in Road Kill Dampner looking for that stuff Rich told me about. Judst dont worry about what engine your using if your putting all aftermarket parts in it, unless you need to order those parts. And if someone wants to race and asks what you have, you can truthfully you dont know or its the original four banger. My friend got wasted by a t-bird who he said "cheated" when the guy said he had the six in it. Lol, lots o' smoke and my friend looking like a dumbass is all I need to say. Luck to you and your car.