This is something I am actually mocking up next/currently . From what I know right now there aren't any turbo manifolds available for the 6g72/4/5 Engine platforms. It is suggested to run the Cali Spec manifolds as they are a cast manifold and more resistant to cracking. The drawback to the cast manifolds is they are not well designed for the optimum performance factor.
I have been looking to pick up a set of the Cali spec manifolds to use for my setup as they will provide a fairly straight forward bolt-on to the hot side I will fabricate. The only issue I am finding right now with how I will be setting up the hot side, is the way the rear manifold outlet faces towards the passenger side. If you run a similar setup as what many have done and run the rear manifold piping to the front where it meets the front manifold there is not much of an issue. The big problem seen with this type of setup is the hot side piping is uneven (basically the front bank will work "less" to drive the turbo than the rear bank). Once the drive pressure is increased (basically exhaust gas flow), in the mid-higher RPM ranges the "unbalanced" flow will be closer to being "balanced" between the front and rear banks.
The optimum setup (which I am setting up for, and shown in the photo below), is to set the turbo over the transmission area in the valley position of the engine (if that makes sense). This will ensure an equal amount of drive pressure (exhaust flow) to the turbine from both the front and rear banks at low-high RPM speeds. With this type of setup, and using the Cali Spec manifolds, the rear manifold is an issue as mentoned above. I would have to make a 180* connection to route the hot side piping to the turbine housing, whether there is enough space between the firewall and engine to do this or not I have yet to determine that. Another option I have to speak with my welder at work about, is if we can cut the rear manifold's exhaust exit, turn it 180* so it exits towards the driver's side and re-weld it (and having it be secure w/o cracking issues).
The second option I am looking into is fabricating a tubular turbo manifold for the front and rear banks so both exit on the driver's side. We could just do a "log" style manifold which would have the 3 exhaust outlets from the heads dumped right into the collector, but this brings up an issue with excessive heat by the time it reaches the manifold's exit. The book "Maximum Boost" details manifolds of this nature to have excessive heat from the number of exhaust gas pulses per cycle. The design of choice for what I would need is seperate piping for each exhaust port until it reachs the outlet. If we cannot make the cast manifolds work out, then I will need to begin fabricating the tubular manifolds. I still have my stock exhaust manifolds (which are the crappy stamped design), so i would cut the manifold "piping" off the flange and use that as the base for the tubular setup.
I have more to look into and mock up to see what will work best, but I think for my specific setup the tubular manifolds will be the route I end up going. For others doing a turbo setup, it really depends on what you want to acheive/spend for the manifolds. You could easily use the Cali Spec manifolds and run the piping how I described in the beginning, or if it is found to have sufficient clearances you could easily make the 180* bend and setup as I am planning to. using the Cali Spec manifold would definitely cut out a large cost factor as well. The amount of piping needed is also far less, and it would be nearly a complete "bolt-on" deal then.
Sorry for the "book," but hopefully that helps and answers any quesitons
