In C6-1G, C6-2G, C6-2M, C6-4M, M1-5M and M1-6M Districts, the City Planning Commission may permit modification of the requirements of Section 15-21 or paragraph (c) of Section 15-021 in accordance with the provisions of Sections 74-711 (Landmark preservation in all districts) or 74-782 (Residential conversion in C6-1G, C6-2G, C6-2M, C6-4M, M1-5B, M1-5M and M1-6M Districts).