Jay, That is my understanding. SS and SS/GT have to have OEM style housings, i.e., the stamped/formed housing and the dome style rear cover on a Ford 9". The modified stock and modified classes can use a fabricated housing made up of sections of plate.
I'm not sure I understand why a fabricated housing would be any better, but it surely is more expensive!
F.J Smith's website shows the different ones he makes, with one specifically identified as S/S legal.
http://www.smithperformance.com/catalogue_rendhous.html