Kinda depends on your mods, but I'd stick with motorcraft plugs, (my second choice would be NGK); you should have the 21lb injectors which should be fine unless you've got some kind of F.I. system going on; and oem on the coils. But if they look to be in good shape, and you don't have any missfires, I wouldn't worry about it. Oh and don't expect a ton of power from LT's even with a tune. I'm not saying its a waste, but same as the injectors; not really necessary unless you're F.I.
*Note* oem coils can be expensive, so I went to a local Junk yard and pulled like a dozen of them for cheap.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***