There is no simple way to do this. Currently there are no harmonized metro codes connecting years before 2000 and after 2012, though IPUMS does have plans to extend MET2013 back to 1990 in the next few months. Please see this forum post for some ideas on how to construct a linkage yourself. At this point there are no plans to harmonize back to 1980, but I have passed your interest in this along to the IPUMS USA geography team as they consider future updates.