I remember from somewhere in the History of Middle-earth series that Tolkien had a grand plan for the Silmarillion that, if he had completed it, would have amounted to more than one volume, so I wouldn't be surprised if he had shown this to Clyde Kilby.

There is an article by Charles Noad in Tolkien's Legendarium (ISBN 0313305307) that tries to reconstruct Tolkien's intentions for The Silmarillion using details given on title pages of the various manuscripts described in The History of Middle-earth series . This is what he came up with:

Quenta Silmarillion

Concerning the Powers

The Great Tales
The Lay of Leithian
Narn i Chin Hurin
The Fall of Gondolin
Earendil the Wanderer

The Later Tales
Of the Rings of Power

The Tale of Years
Of the Laws and Customs among the Eldar
Dangweth Pengolod
Athrabeth Finrod ah Andreth
Quendi and Eldar