With the narrow-ish nut, I assume that you won't be doing the bizarre SB1000 parallel string wide-nut-narrow-bridge thing? You could still no a narrow bridge - Hipshot makes ones down to 16.66mm string spacing, the original Arias were apparently 16mm. That said, one of my regrets on my latest build (the short scale Talman-style one) is that I spaced the strings too narrowly.
I don't think the width of the neck should dictate your fingerboard radius, just pick one you like on your other basses. I personally prefer a really small radius. You might just want to use whatever radius your slotted brass nut uses, so they will match without much adjustment.
Yeah, the 80s SB1000s had, I believe, 16mm spacing nut to bridge. Mine actually spreads "normally" much like a Jazz. I need to find my calipers to get an accurate measurement. As for my Aria's radius, I want to say 12". And you are right, I pretty much decided on radius when I ordered the nut. If it is a well-made part, I'll just go with that.
Wow! So many details! I couldn't imagine trying to keep all of this organized in my head....