if i were u id keep my 289 heads but just get them ported and polished if u want, maybe resurfaced, its alot easier and u will have plenty of power, i do and didnt even get them ported. the edelbrock manifold u got is good but i would have just gotten a performer dual plane manifold( i love mine).headers would be the first step before u upgrade too much so ur engine can breath better. next i would go for a cam but not too big, mine has like a .448 and .472 lift. as for mufflers, every 5.0 mustang has flowmaster, if u wanna be like everyone else then be my guest. for now i just have some generic turbos from JC whitney and i get plenty of complements but i really dont think it makes much difference as far as power, but what do i know. anyways, get headers they sound great. and a bigger cam. should make a noticeable difference.
As far as the bolt pattern is concearned, the 351 headers should bolt right up. If you use shorties, you'll be fine. If you use full lenght headers, they are going to hang down 1" lower than normal. This will cause them to drag over most speed bumps.