I wonder if it might taste too burnt with that much black, but I think it will be good. Nice idea!
I submitted a Porter that was straight outta Ron Pattinson, and all the judges said it was too roasty for a Porter. So either old skool black malt was somehow less roasted or that's just how Porter was in the late 1800s.
From what I recall of the Durden Park Beer Circle book, historical porters were very roasty and very bitter/hoppy.