The 4.6,5.4 and V10 all had spark plug issues.As berge said these are not as common as you may think.Millions of these motors have been built even if it was only .5 of 1% this would be a high number of incidents.The heads do NOT have to be replaced,there are some good repair kits that use a long 3/4 inch stainless steel thread insert that repairs these to stronger than original condition.If the plugs are not left in there for eons and torqued properly there is very seldom any issue.Two areas to look at carefully on these motors is the oil pan,they have a habit of rusting through when used in snow/salt conditions and not being washed underneath,the engine must be pulled to remove the pan,the exhaust manifolds,especially the right side,the studs rot off warping the manifold until it cracks.again keeping the salt of them usually increases their life span.A good fix is Gibson stainless headers with stainless hardware,the $500 cost is well worth it.If the oil pan is in good shape keeping it that way is no problem,just wash it and keep paint on it,there really isn't much that can be done for the manifolds except replacement before they let go if they are severely rotten.