For my CIS-to-EFI conversion I decided I needed a porsche 911 airbox to hack up. I want to keep my existing CIS system incase I decide to sell the car in the future. I don't think I'll ever sell my 911 but you never know and I don't think anyone would buy it with a custom EFI system.
I found an airbox listed on ebay with six runners. It was advertised as "fits 2.7L and 3.0L". I picked it up. It arrived today, however there is a problem. The airbox does not fit a 3.0L engine, nor do the runners. This box and runners have a smaller inside diameter. The intake runners  have an ID of 1.3". My 3.0L runners have a 1.5" ID. This is 25% less area. I think this will choke the engine, and so I cannot use it. It must have come from a 2.4L CIS engine.
Here is a pic of my 3.0 L air box.
Here is a pic of the air box I got on ebay. It came with a pop off valve. All other dimensions seem to be the same, except for the diameter of the outlets. Damn!
Maybe I'll sell this one, and make my own air box. I'll have to think about this.
I also picked up a throttle body from a 5.0L mustang. The butterfly is exactly the same as my 911SC (59mm). It has the TPS sensor on it and a idle air valve. Unfortunately the solenoid which drives the idle air valve was shorted. I'll have to find another one. The screw holes don't match up with the throttle body of the Porsche, but I can easily make an adapter plate.
Update Feb 19,2002: I did some reseach and it turns out that the smaller diameter runners are not from older 911s, but are indeed from a 3.0L 911SC. after 1979 Porsche added smog gear on the engine which reduced the lower rpm power of the engine. Porsche brought back some power to the low end by reducing the diameter of the runners. This increases the volumetric effeciency of the engine at lower rpms, but may take away some at higher rpms. Since this is a street car I prefer to have more power at the lower rpms. I will go ahead and use this airbox and runners for my CIS-to-EFI conversion.
Stay tuned I am now working on my fuel rails.