Sorry to be late the discussion!
The Turbo uses the same fuel distributor as non-turbo Lambda cars (Bosch 0 438 100 032). This is the easiest to use assuming you plan to use the entire Lambda system (oxygen/"lambda" sensor, modulating valve, electronic control module, wide open throttle switch, etc.) as it was installed on 1978-1980 USA spec 99 and 900 turbo cars. If you are planning to try to achieve higher than stock output from your engine, I'd suggest that you install a wide-range air-fuel ratio sensor so that you can monitor the actual operating mixture. This fuel distributor is at the outside of it's flow capacity in this application I think which may have accounted for a lot of engine failures when people raised the boost - they ran too lean.
The six-cylinder unit with 2 ports plugged was simply a way of getting a non-lambda fuel distributor with higher flow than the 005 and 023 4-port units (always try to use an 023 for not-lambda cars - it has a stainless diaphragm that won't rust and get holes in it). The six-cylinder unit will not necessarily be any better than the lambda 032 unit for fuel flow, I would not really think of it as an "upgrade", it's just non-lambda with the right fuel flow for the engine.
In an engine with 10-12 psi boost (probably the most that's reasonable with this engine given the spark control of the distributor) I would want to be running no leaner than 12.0:1 air:fuel ratio under boost, and it wouldn't hurt to be even richer than that. There is a "boost enrichment" Control Pressure Regulator 0 438 140 085 which may allow you to run richer than the stock system (I have one - thanks, Jimmy!! - but haven't tried it as I want to get the air-fuel ratio sensor installed first).
I definitely agree with the advice to get whatever Control Pressure Regulator and Fuel Distributor you will use rebuilt and then get the fuel system well-sorted before you start running the engine under boost.
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