Ok I went out and measured. From inside tip edge to inside tip edge is 20 inches exactly. If I can find my camera I'll get a picture from where I measured from if you don't understand.
Okay, it's time to toss my pink wrench into the pile here and make things interesting.
I measured the one here that my customer indicated is from a '69 Polara and it's only 17 1/2" across the center to the outside edge. No pics though ... Alan gets to see it first and then I can share it with the rest of the world. :-D