I want to know if an oilpan from a 1971 Riviera 455 will fit a 1972 Skylark 455. Are they the same.

There were only two pans and pick-up tubes for the 455. There was a rear sump pan used in all GS Skylarks and a mid sump pan that covered many other applications. If the Riviera has a rear sump pan, it will work on the Skylark 455. I am not sure what pan the Riviera used.