How similar is the '66 Valiant VC compared to my '66 US Dart?

I don't know what the "VC" is in your description. In general, all the parts are the same across the company, not just car lines/body style platform. Though certain models recieved bigger front end parts mostly due to bigger engines or performance packages. I'd recomend the bigger beefier parts when you can get them over standard parts if you can. They last longer.

I also here good things about that place you want to order from.