Hi Eric
Scribing that line will definitely help. I've had a look at the pictures and the 1/48 copies I have of the 27 (from the Tamiya F-1117 kit) and the 28 (Academy). The fin assembly that mates to the bomb casing should extend beyond the fins. I think that's what is throwing me about your conversion. At the moment, the scribed line mid-way through the fins looks like the start of the fin assembly.
I like to paint the fin assembly different colours from the bomb casing as it adds colour and interest, so to me it's important they're clearly differentiated. As you say, it's a WIP so you can scribe the flange.
Looking forward to seeing the finished Nighthawk...