Jim D and Gene, as always, are correct. Your '72 Impala needs the quad set of PowerBeams. They will say "Guide" on them and have a circular pattern in the center of the bulb. T-3s ended in 1970. PowerBeams first appeared on the 1970 Camaros in February when they came out late as "70 1/2"
I do have two matching high beams on ebay. http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=280552457966&ssPageName=STRK:MESELX:IT
Contact me about them if you are interested.