Get 100ft. 1.75x.120 DOM is overkill. For a 911, use 1.5x.120 DOM. Almost everything else, use 1.75x.095 DOM. Do not use anything but DOM, unless you are prepared to normalize the welds on ChroMoly. - we only use DOM, and that is for everything from Grand AM to PCA.
Put lots of research into the design - there are some questionable decisions exhibited in a couple of the examples shown. There are lots of discussions on the major Porsche forums about good cage design that will shed some light on design do's and don'ts. Proper design is very important, along with proper materials and tools - a cage, for better or worse,
becomes the car. It will either improve or degrade the vehicle, and it is more difficult to remedy than a failed engine or transaxle.
Also, keep in mind that a proper cage is a system - involving seating position and height (and proper mounts) harness angle and anchoring, window net positioning and mechanism, mirror positioning and blind spots, ease of entry and egress (particularly in the event of fire), etc., etc. ad nauseum. Base plates can even improve the chassis when well thought out (and can be aetheticallly pleasing as well
)
Um, that's all I have for you right off the cuff, have to get back to earning a living!