First, Reg, your car looks really nice!
About the offset and width, you can easily get the 15x9 -15 to fit under the fenders without flares. I've got the Rotas 17x9,5 -19 to fit under the rear fenders with some minor modification. First I run 225/35-17 tires, AZcar coilovers, rolled the fender so they have a slight angle outwards, a set of camberplates (not wild neg camber but about 2,5 degrees) and a set of adjustable LCA's. The project isn't finished yet and my final wheel setup will be Rotiform race wheels with 16x10 -5 with 235/45 or 17x10 -5 with 235/35 tyres on. That would fit perfect for a form and function setup.
It was poking about 5mm outside the fender with just coilovers, 225/35 tyres and rolled fenders, but I had no rubbing issues since the tyre was leaning with good clearance to the fender. Stiff springs (I run 375lbs/inch) and adjustable shocks is required since the ride have to be hard to do this.
I know many of you on this forum don't like to see cars that chooses form over function, but its still a hybrid z. And you have to agree, a low, wide and classy done Z is the best pinup model;)