FPP-1885 is Oil Pan Gasket, 1-Piece, Rubber/Steel Core, Chevy 1955-75, Thin Front Seal

FPP-1880 is Oil Pan Gasket, 1-Piece, Rubber/Steel Core, Chevy 1975-79, Small Block, Thick Seal

or two piece

FPP-1821 is Oil Pan Gasket, Multi-Piece, Rubber/Steel Core, Chevy 1955-79, Small Block, Kit
(two thin and one thick seal)

Turn your pan up side down on a flat surface and measure from the flat surface to the highest point of the front opening. If it measures 2 and a 1/4 inches then you need the thin seal, if it measures 2 and 7/16 then you need a thick seal.