TB, I'm not sure the Hiss case is a very good parallel.

While, if memory serves, there was no corrobating witness to Chambers, there was a great deal of evidence, mostly documents that Chambers said he received from Hiss.

I believe that the "he said, she said" aspects of the case were only a sideline and most of the case was consumed with technical testimony centered around Hiss's typewriter and whether the documents in question could have been typed on them. There is no equivalent evidence in the Corleone case.

Incidentally, I don't think any suspense novelist could have invented a better name for an accused spy than "Alger Hiss."