The 66 front crossmember is cosmetically different. The 64-5 was a tab welded on. On 66 it's molded into the frame.
Interesting, Walt, I have a frame here that was from an original '66 Skylark, and it has the welded on front piece. I was always curious about that since my other 66 frames are the 'molded' style. I had assumed that it was an assembly plant difference, just like I have seen fasteners differing in 66 models that were original. But maybe this frame here was a leftover from 65? And maybe assembled onto an early 66 car? who knows, but it is interesting!
You might try Joe Vega, aka "azchevs". Believe he is thin on first gen A stuff, but he may know of a possible Arizona source. Aside from the desert and hidden valley yards. Good Luck.
The frame I have is from a '66, but it does have the welded on front piece on the main crossmember like a 64-65 noted in Walt's post. It is a rust free really nice desert frame, unfortunately, it is not a convertible boxed frame, it's the 'open' or 'C' channel type. Let me know if you are interested. Located in AZ, might even be able to deliver to CA Mike