Generally, headers are considered to be beneficial, but they can also be trouble as they might age. But they look neat and can give some better performance at the mid-upper rpm ranges.
It used to be that to see a set of headers without some ball pein hammer "clearance notches" meant that you had the application they were designed for, which seemed to be rare. The header gaskets to the cyl heads might have improved, but the old ones were not long-lived, especially if the flanges were warped, although quality control on the headers has seemed to improve over the decades as many smaller companies were acquired by larger companies. Getting a "thick gauge" tube is important on all accounts and can take some of the distinctive tinkle out of the sound under the hood.
2.25 to 2.50" pipes out the back would be good. The larger the pipe, the harder it is to bend over the rear axle without collapsing it in the process. Which is where mandrel bends come into play. PLUS you have to clear the rear shock absorbers, too!
In reality, unless you're going to inrease the horsepower greatly, keep the stock manifolds and improve the muffler flow. Unless you particularly like glass packs, getting one of the Street Hemi mufflers would be good. Or the '72 Imperial mufflers that should fit in all respects other than pipe diameter as they are the same C-body case size with larger pipes going into and coming out of them.
IF the system is basically stock, the inlet pipes should already be 2.25" pipes, which would then mean you'd just need mufflers (with the larger pipe outlets) and the matching rear pipes. Remember, too, that "power" does NOT need to be loud.
Just some thoughts,
CBODY67