Can you post up some pics coz I think you've been jipped.
The standard rear housing A/R on an RB20 is 0.63 so I don't know how you matched this up with your figure of 0.48.
If what you have described in the last part is the same thing I'm thinking of, it's a bolt on internal wastegate plate that garrett made so that the GT30R, 3071 and 3076 could be internally gated. This was the most dodgey setup on a turbo I had seen and I told my rep that it would not sell. I was right.
There are so many turbos to chose from and it sounds like someone may have given you the wrong advice.