Kim,
There's a lot of controversy regarding that manifold due to some inaccurate published data. I can assure you, that intake was used on the '61 340hp 348 and the first '61 409s. Most people believe the '59 story, but I'm absolutely certain my info is accurate.
Also, the 586 heads are '61 409. Quite rare and valuable.
Hope this helps.
Verne
