As mentioned, 66 300 has many parts specific to 66 and specific to 300, and the taillights and front cornering lights (and associated trim) are part of it, those are hard to find and expensive if the seller knows what he's got.
The trim on the lights is diecast because it had to be, that appearance/cross-section couldn't easily be rendered in a stamping from aluminum or stainless like the rest of the trim. And it was cast from potmetal for cost reasons, but I suspect every manufacturer used potmetal, these cars weren't meant to last 20+ years and the accountants overruled the engineers from a long ways back.