Hi Paul,
the wheels used for the conversion do come from GSX-R 750 or 1100 of the years 1985 - 1988 I think. Suzuki used internal codes for these bikes like GU74C (1100) and GR75A (750).
Not allways they are used when selling parts, but it is defenitely the way to identify if its the right rims. You can see pics of the different models suitable here:
http://gsx-r1100.ruhrpott-gixxer.de/
http://gsx-r750.ruhrpott-gixxer.de/
please notice: the 6-spoke wheels (which look quite similar to the Oscam wheels used on the 1000 SFC) are the right ones. 3-Spoke wheels are most likely 17". All rims where the spokes are not straight are not 18" either I think.
HTH