That's right JT, ALL of the 351C 4V air filter assemblies, 1970 - 1974, consisted of the large 18" OD base. There were variations for different types of cold air induction, ram air seal, shaker asemblies, chrome lids, etc; but all used that same size base.
The air filters were either a Fram CA324A (13" OD x 2.75" tall) or a Fram CA340A (13" OD x 2.5" tall).
Your 16" OD assembly is not the original unit. Its something a previous owner threw on there to make it look stock, pass smog inspection, etc.
cowboy from hell
