273 to 360 swap. same converter?

Now, what is the purpose of this adaptor ring? If you buy the aformentioned flexplate, What does the adaptor ring adapt? I measured the cranks from both engines and although the newer one had a larger inside diameter than the old one they are basically the same. I don't think there would be a problem if the snout of the older converter didn't fit in the crank as snug as the newer converter. As long as there wasn't a problem of the converter fitting in the end of the crank properly it should work if I just bought the b&m flexplate. tell me where I'm wrong.