I think I found it. WFB had mangled the quote in his recitation, so I wound up doing very limited searches for only one or two words or phrases. Here's the actual quote and source:
> Insofar as a nation is content to practice its doctrines within its own frontiers, that nation, however repugnant its ideology, is one with which we have no proper quarrel.
(Senator J. W. Fulbright, chairman of the Foreign Relations Committee, in the Senate, March 25, 1964)
Buckley translated this in his conversation as:
> The United States government (as opposed to the United States people) should have no quarrel with the policies of any other nation, no matter how odious, so long as it does not seek to export them.
The source was here: Excerpts From Fulbright's Address Urging United States Foreign Policy Changes