If you're still looking, here's a decent '66 for cheap: https://seattle.craigslist.org/tac/cto/d/1966-buick-riviera/6683825256.html
I don't get on here much anymore and just stumbled onto this. I hope the 67GS is serving you well. My preference is mostly sentimental. My father bought a new 66GS Riv when I was 14. At 16 I had a car, but sometimes had a chance to borrow his and have my way with it. I do prefer the appearance of the GS nailhead engine with the finned aluminum valve covers, and chrome "Wildcat 465" air cleaner cover. Performance probably very similar. Under tail lights are silver not body color. Very subtle differences that almost nobody would notice, but to me its not the same. Like the emblem under the glove box. On the 66 it's wood, on 67 it's plastic. I could go on and on. They are both great.
