Exceeding qt 15 diameter capacity: How to do it?

I've got to quote an aluminum clutch housing that is 16" od, 5" id, and 4" thick. Mostly hollow inside, like a big flat bottomed bowl. Was a casting they don't have anymore.

Has anyone cheated the max turning diameter by building special boring bars that attach via the od turning holders? Choking up on the od turning holders so that they cut a larger diameter than usual?

I might be able to get away with milling it complete, but it's a great lathe part, with milling after.

Advisable? Foolish? Does anyone have any good tips? What say you?

Thanks,

Grant