I'm the owner of the #919 X1/9 pictured above. That was at Autobahn with me in the car. The high back seat we chose was higher than the targa bar. So going over the targa bar with the cage was just easier especially since Lemons was requiring a rear window in a mid engine car at the time. So the easiest way was to just build the cage high enough to 45 the rear supports over the top. I have a many pictures of the original build including the cage. Contact me directly if you want me to send you some pictures. Also if you are looking for parts I can help you there as well. Just let me know.
If you plan on using the stock drive train your in for a lot of heartache. I have owned an X1/9 since I was 17 (I'm now 55) and figured racing an X1/9 in Lemons would be no problem. Many times we would make it through day one with no issues just to blow up on day 2. We did win IOE once, and were in contention to win class C a few times, but always walked a way a bit disappointed after one failure or another. A few times in the last 1-2 hours.
The best option for an X1/9 in Lemons in my opinion is some sort of an engine swap. Splatos ran an Alfa v6 with success. Although the builders of Splatos were Alfa gurus. You can google "Splatos" Lots of information on line regarding this car. I was able to help provide them with some parts after there unfortunate incident at Road America when Lemons ran there.
I have been meaning to sell the #919 car above, but just never got around to it yet.
Good Luck with your build.
#333 2001 Ford Escort ZX2
#919 1974 Fiat X1/9 (Autobahn July 2014 IOE)