I'm looking at Garrett turbos and am in a similar position. AFAICT -> http://horsepowerinabox.com/HPIAB2/category12_2.htm the 5th and 6th options are the only garrett turbos I have found that have the 6 bolt housing that you require.
I'm not interested in a rebuilt turbo, what other options for new direct bolt in turbos are there for RB20/25?