For the archives and the searchers: I couldn't find anything narrower than a 16x7" with the vendors I was considering...so I'm a tad richer, but the car's still got the stock wheels.
A 16x7" with a zero offset would clear the struts in the rear, but stuck way past the fenders ... as I told the tire guys, I wouldn't let my wife go out looking like that ... not the car, either. 16x7 with an offset, like 35mm or 40mm (which is near the original offset on the 15x6") was dangerously close to the rear struts.
So, my take is that you probably will want a 16x6" if you want to go "plus one" on your Capri.