The silver king is the xtra beefy version of the 12+ weight (ie built for the biggest blue water tuna/bill fish, Giant Trevally and record size tarpon) CCF-x2, which is Nautilus’ top of the line reel series, so if any fly reel is beefy enough for downrigger use, this has gotta be it. Spool warp from mono still might be an issue, so backing or braid with a mono top shot likely the best approach.
Beauty reel, if I had a fly fishing application for it I’d jump on it!
Cheers!
Ukee