The 65 and 66 pontiac big car platform was the same body between the catalina, parissian, bonneville,ventura and 2+2.
The basic changes between the two years is the front grill. 65 is rounded and kinda dull and 66 is pointed and very "moving" in form. Some interior differences as well.
I've been a big pontiac car guy for years. I've got a 66 2+2 with a 421 big block pontiac. 389 was the baseline for American Pontiacs. 389 is a great running motor and will outlast most other motors be it gm, ford or mopar. I've had several.
I love big cruisers. That's why I joined here with the hopes of finding a 70 gt. Or maybe a polara or monaco.