the CA18 powered Silvias and 180sx i BELIEVE had the same size rotors etc as the USDM 240sx.
Easy way to confirm that is if anyone knows if the 240SXs had smaller diameter rotors than the SR20DET powered silvias..
I know the CA ones are smaller tha the SR powered S13s.Atleast according to boost cruising..
http://www.boostcruising.com/silvia/sr20brakes.asp
Confirm it first though, or maybe see if any friends with 240s have a rotor lying around..