Chassis, engine, engine compartment bits - the relatively common '66 is a good source for resto parts.
Racing parts are gone - there wasn't going to be a GN '66 Mercury, although, in my secret heart, I wanted to build one, and IIRC, my Show-n-Go build had one made of sprue (at least as bad as it sounds) For uber-geeks, the Merc interior does NOT have convertible doglegs, betraying the client decision not to do a promo of same. (My father had an S-55 company car convert, which was very nicely trimmed out, but, was a slug compared to his '64.) This is a pretty close fit for a '66 Galaxie, and relieves the need to knock out the doglegs for a really accurate NASCAR/ARCA build.
The custom parts in '66 were different as well much better executed than '65, which was pretty fussy looking. Been meaning to use the parts on a "SuperMarauder II" show car idea that's been bouncing around my noggin for a while.
My original SnG release (ha, I just realized what that internal handle probably represented!) had excellent Hurst mags. They survived all these years, and I am now pouring replicas for my own use.
I can't speak to the annual '66, which come to think of it, is a pretty rare duck, not nearly as common as the re-releases over the years.