Well I ment to say, not as wide as with a stock disc-setup.
Going to discs usually adds (unwanted) trackwidth (on an A-body).
I went from stock A-body discs to C-body discs and I recall to find the track to be a tiny bit narrower. I don't recall any numbers because I've been changing wheels and tires during that time aswell.

Yes. When using C-body rotors, the track width decreases about 1/4" per side when compared to '73 and newer A/B/E/F/M/J/R body rotors.