I would only use motorcraft. You can get them from any dealer. I believe the stock ones are SP580 and should be gapped at .032
As long as the coils aren't frozen onto the plugs, it's relatively easy. My first SHO I bought with 100K miles and the coils had probably never been removed, they were an absolute pain in the butt to get off. Test the front ones, if they come easy you should be fine. If they are stuck, the back ones will be a load of fun.
For the back plugs, it helps to remove the bolt holding the rear charge pipe down. It's about 6 inches behind your front furthest right cylinder. Once that bolt is off, you can lift the charge pipe off the stud and it will give you a little more room to work. A variety of different length wobble extensions will make the job a little easier as well.
When you tighten the coil bolts back down, do not over tighten them or you will strip them. The only way to fix that is replacing the valve cover, so baby strength tight is good.