FYI to anyone who cares.
I pulled the rear wheels on my 92 (SN95 axles, Cobra brakes) to measure the width. Using a straight edge on each rotor flange and a tape inbetween - the width is almost exactally 61". So I pulled the rear wheel of my 03 Cobra. I supported the weight on the lower control arms to make the axles pointing exactally straight out (maximum width) - almost exactally 62.5". 3/4" wider per side. That would put the bulge of the tires on the 92 slightly ouside the fender. I can see where somone may get a narrower measurement, if the car was supported by the body with the axles shafts arched down it would make it seem narrower or if the rotors were not on.
What wheels are being used with this conversion that don't poke out past the fender making it look funny?