Are 55,56,57 Chevy belair frames the same

55 and 56 are the same, not sure about the 57 as it was a major body change, This is part of what I found: From at least 1936 through 1958, GM used at least four different designations for various bodyshells/platforms including the A-body for Chevrolet, most Pontiacs, and the Oldsmobile Series F and Series 60, the B-body for the Pontiac Streamliner Torpedo and Streamliner, the Oldsmobile Series L, Series 70 and Series 88, the Buick Special and Century, the LaSalle Series 50 and the Cadillac Series 60, Series 61 and Series 63, and the C-body for the Pontiac Series 24/29 Torpedo, Oldsmobile 90, the Buick Roadmaster, Super and 1958 Limited, the LaSalle Series 52, and all remaining Cadillacs except for the Series 90, Series 85 and the Series 75 which were built on the D-body, along with all remaining Buick Limiteds. For the 1959 model year, the previous A and B bodies were built on the new B-body[1][2] that lasted until 1996. The A-body designation would be resurrected by GM in 1964 for a new series of intermediate-sized cars including the Chevrolet Chevelle, Pontiac Tempest, Oldsmobile Cutlass and Buick Skylark. The GM B-body had at least 12 major re-engineering and restyling efforts, in 1937, 1939, 1941, 1949, 1954, 1957, 1959, 1961, 1965, 1971, 1977, and 1991; along with interim styling changes in, 1942, 1969 and 1980 that included new sheetmetal and revised rooflines. The platform was downsized in length by approximately 10 inches in 1977 and reduced in weight by an average of 800 pounds.

It is the same thing with Chrysler cars from 1949 to 1956 they share the same frames with major restyling and the cars weren't as tall in 1955 than in 1949.

Are the 93 Buick road master 4 door frame the samme as a 78 bel air 2 door ?

The tri 5 frames are mostly the same and can be adapted to different years. You may need different mounting brackets or shackles.