I'm thinking of picking up some 300ZX rims, and I'd get an extra to use as a full size spare... will one of these fit in the spare well?

 

I don't think so. The other day I put one my stock 15" GXE rims (15x6) in the spare tire bin and it barely fit. Thus, I don't think a wider rim would fit.

 

You forget something,

even though the rim is wider, the tire's profile is smaller, so in the end, the total diameter should be exactly the same as your 15" tire and rim.

I have fit my 17*7 using 225/45/17 inside the spare's space. The cardboard cover does not close over it anymore,(the tire sticks up about an inch or so), but the tire sits inside OK.