John,
You can just manage to fit a 577 NE into a Ruger #1, a cheaper way to have a bigger bore, but for a 600 I think you'd either have to go with a double, or maybe one of the really big FBW single shot actions, if you could ever find one.

Or maybe we could get someone to hang a new barrel on a H&R Handi-Rifle in 600 NE?

Regards,
David.