SP515 is it.
Dealing with stripped heads on 3 coil hold down bolts. I have 5 plugs marinating in penetrating oil. Heading out for opening season dove hunt, will try something else on bolts tonight

 

I ordered 515's from amazon and they sent 546's... Also after chasing a slight rough idle I followed one of F150torqued post from the past today and brushed all the spark plug holes with cleaner and blew them dry, re installed everything and miss is better but still there, so I'm still on the hunt.

 

Tackling plugs in the morning. Been soaking since last night. Going to let engine warm up and then we will see how it goes.

 

I am late to reply to this, but you got the correct plugs (515's) for a 3 valve. 2004 was a transition year between the old body style F150 (1997-2004 with 2V) and the new body style (2004+ with 3V).

I replaced my plugs about 3 months ago on my '05 3 valve and used the 515's. I hadn't heard of a possible switch to the 546's? Most tips have been mentioned already, I did pull the ECM to better access the back plug on that side. I've done mine twice now and haven't broken any yet, but I did the first plug swap way early when the issue first started appearing on some trucks even though I was only at 30k at the time. I bought the Lisle tool and ended up not needing it

Good luck

 

Well I got the plugs out without any breaking.
Not sure if Sea Foam helped, but I did it. Used Free All penetrating oil because that is what was available. Letting the engine warm up was definitely a big help. Did a lot of "Please Lord". Got a few other things to do before returning the truck to owner.
1) Heavy on Sea Foam
2) Soak overnight with a quality penetrating oil
3) Warm engine
4) Pray

Thanks for the help guys