Hi Walt,
Don't know if you saw the apparent interchange information contained in the air filter description. Here it is:
This is a new dual quad ( 2x4), or tri-power (3x2) air filter correct for 1964, 1965, and 1966 Buick Riviera Gran Sport, Wildcat, and Electra with 425 V8 2x4, 1958, 1959, and 1960 Cadillac Eldorado Seville, and Eldorado Biarritz with 390 V8 3x2, 1957, and 1958 Oldsmobile Dynamic 88 Holiday J2 Rocket V8 371, 1966 Cutlass 442 400 3x2, 1956, 1957, 1958, 1959, 1960, and 1961 Chevrolet BelAir, and Impala 283 V8 3x2, 1958, 1959, 1960, and 1961 V8 348, 3x2, 1963 396 3x2, 1962, 1964, and 1965 V8 409 2x4.
I can't assure that this is correct information, but it gives us more applications to research. Probably another dead end, but.......
Chuck