Which direction to turn ?

Hey guys I own a 1999 Ford Mustang, which I am having Cat issues with. It has burned out two sets so far. This is a V6 standard with no aftermarket parts on it. I have replaced the plugs at one time or another and regular oil changes. I have seen an article that said fluids are getting past either my seats or plugs and getting into my cats and this is what is causing my troubles.

My question is prior to changing out the cats for one last time.
Should I
1 Change wires
2. Change plugs
3. Rebuild heads
4. ????

The motor runs strong and get great gas mileage. I replaced the front O2 sensors after the first set of cats but not the back ones (it has 4).

Thanks for any advise on this subject
Cigars99