I know this thread is old but....my friend and I are working on getting a twin turbo setup using the EFR-6758 turbos.
We will be building custom manifolds and piping. My buddy is in the middle of drafting up design's right now as to how everything will fit as well as piece together. We also started speaking wtih Full Race this week and they seem mildly interested in this and are inquiring for more data on the project.
We are aiming to do the first TOP mount twin turbo for the Z34. We will know more on the possiblities of a top mount after the drafts are done. This project is slated to be completed about a year out though guys so dont get too excited..These turbos are back ordered by 8-10 months.