Some guides required here!

I've finally managed to finish all the main character sprites, even after freakin' realizing only the first 12 are usable in battles and having to recolor everything. My next step right now is to change certain weapon sprites, that from what I've heard is not as easy to do.

My goals are changing the GP sprite used in GP Rain to Dynamite Headdy's head (sprite which I already have done), so that it looks like he's attacking him with the head rather from using money (whoever has played Dynamite Headdy should know why). What is the easiest way to achieve this, knowing I'm quite bad with anything that's not FF3usME and FF3SE?

I also wanted to change, in a similar fasion, the sprites from all "Boomerang-Full Moon" weapons to the very same head. I realize it must be quite the same thing, but still.

The last unrelated thing would be this. Since GP rain is pretty much gamebreaker at the start, I want Headdy (locke) to stay ability-less until he finds the object that "changes Slot for GP Rain", that once equipped, lets it gain access to the ability, GP Rain that is. Is it possible to do such thing?

Any help is appreciated beforehand. I am aware there are some video tutorials out there... but I'm fairly more comfortable at reading a guide.

Thanks in advance!