Within the SAME generation, things can happen. NOT within the same generation, architectural differences exist, as to where and how things mount. Like the stubframe to the cabin area AND the core support. So, you need something from the "slab" years, '66-'68, generally.
You'll NEED to get outside of normal, high-volume salvage yards, I suspect. Get into the salvage yard circuit which deals with OLDER cars, for example. A little-known-about subset of the salvage yard industry. Browne's Salvage in Sunset, TX is one of those older-oriented locations. He buys and sells cars with other similar yards, too. About a year or so ago, he had a '66 Newport sedan up there. He has a Facebook page.
There used to be a big yard just east of Abilene, TX, north of I-20. Also a larger yard west of Graham, TX headed toward Breckenridge. Not sure what is in Lubbock anymore, but there used to be some larger yards up there.
Enjoy the hunt!
CBODY67