A universal Ford/Chrysler unit should work. I'm running one from Jegs. Works pretty good. Usually stays under 200* even when its 100* out in the summer. If I had to do it over again, I'd go with a 2 row 1.25in tube unit from Griffin.
My $.02; If you go aluminum, shoot for a unit with 2 rows of 1" or better tubes and ignore the ebay 3- and 4-row rads - their extra (and typically smaller) rows don't cool as well, something about reduced airflow through all the cores. I don't know beans about '64 Rivieras, but it was worth my while to research which other make/model radiators cross-referenced when I was looking for an aluminum rad for both my '62 Ranchero and my '71 LeSabre. I've had good experiences with "Engineered Cooling Products", but a quick search doesn't show they make one for your car (but who know what cross-refs?) http://www.speedcooling.com/ I have zero experience with Griffin radiators, but you might want to check this thread on them: http://www.v8buick.com/showthread.php?280405 Good Hunting!
