As a long time owner of a 1966 Coronet 500 with a 1969 hp 440 big block, along with owning several other big block powered Mopars over the years, I hope that you won't mind me offering you another option other than a new set of headers.
Year One sells the complete kit shown below under their part number HP689S which includes all of the parts that you see in the picture for about the same price as a
good set of headers will cost you and these reproduction hp cast iron manifolds will bolt
directly onto your 383 in your 66 Coronet with
no modifications needed other than to the exhaust piping from the exhaust manifolds to your mufflers and unless you have some aftermarket parts in place, it won't matter which factory options your Coronet may have, including factory power steering, factory power brakes and in most cases, factory a/c. My 66 Coronet has factory power steering, aftermarket power assisted front disc brakes and will have a/c as soon as I reinstall all of the underhood a/c parts that I removed for drag racing and my hp cast iron manifolds have no trouble living with all of these options. These hp manifolds will also last for
many years without the issues that most any new set of headers outside of expensive custom made headers will most likely give you.
As far as how these style hp manifolds sound, with the custom 3" hp exhaust system that I run on my Coronet, folks will swear that I'm running headers until I show them otherwise and I've been known to rattle my neighbours windows on occasion.
Performance wise, my car has run some high 12 second quarter mile passes in California's cooler weather with a set of the original 1969 cast iron hp manifolds in place and many low 13 second quarter mile passes in the hot weather here in Phoenix. All of this done with a fairly stock and heavy, all steel car.
I don't know how your 383 is set-up performance wise, or how you use your car most of the time and I'm not trying to change your mind about running headers, but I did want to share another viable option that has worked very well for me on several cars over the years.
Richard