Because its for his wife and there's no sense in wasting that kind of money(additional modifications to make it work) and time on the cheap ****ty headers with huge ass primary's that are not needed on the majority of 3800 S/C setups. Bigger is not always better but most of you guys believe that. It's rather humorous actually. And headers are not the best investment, a trans is :P because it will eventually go out. Then most will demonstrate Einstein's definition of stupid and replace it with a stock one.
Vandezand- Get the plog and call it done. No sense in changing out the rear manifold as stated above. As far as cracks go I know the ones that have the most issues are the turbo guys who do not use the stainless steel version. Since the wife's car is more than likely a daily and nothing more I'd go with the plog as its most likely cheaper than an OEM replacement or a tad more.