Not to be contrary, but do not fear changing the spark plugs on a '99 V10. If it is done correctly, using the right parts, and torqued properly and rechecked again after several heat cycles, there is little to fear, even on a '99 V10 which does have fewer threads than later models of the V10. Leaving them in for 100K, even though it is the factory recommended interval, is too long and will create problems removing them. The plugs are probably fine but the boots are likely dried out and cracked which will lead to COP failure in the future. I'd say change the plugs now, replace the plug boots, and coat them with dielectric grease to help them seal.
It is very important that you physically check each of the exhaust studs on both manifolds. Replacement is difficult and time consuming and expensive if you have a shop do it.
