I think I would repair what you have. The 2 valve 5.4s last a long time typically.

 

For the head I would repair it with lock and stitch. It's what all the high hp guys are running. It is also the best way to repair the threads. You might as well do all 8 if the heads are going to be off. Save yourself from having to do it while the heads are on. Especially for the rear plugs.

http://locknstitch.com/sparkpluginserts.htm