No - there have been a few other elections where this figure has been broken or equalled.
The most recent was 1968, where states totalling 333 electoral votes voted for Nixon more than the national average of 43.42%. These were the states won by Nixon (301) minus Florida, North & South Carolina, Virginia, and Tennessee (57) plus Washington, New York, Connecticut, and Kentucky (89).
Before that, the election in 1944 equalled this figure of 320, slightly more impressively, as there were only 531 total electoral votes compared to 538 today. The states were all those which voted for Dewey (99) plus Michigan, New Jersey, Missouri, Pennsylvanis, Maryland, Idaho, Illinois, New Hampshire, New York, Massachusetts, Connecticut, Oregon, Minnesota, & New Mexico (221).
Before that, as the number of total electoral votes begins to dwindle, it becomes fairly impossible for this record to be broken in terms of raw votes. Nevertheless, it was broken in percentage terms, for example in 1892 (64.8%), 1888 (64.0%), 1880 (61.8%), and 1876 (60.4%).