How tall? We are all 6'1"1+ and we fit fine (88 Trans Am). You could cut the roof off and the re-attach it to the halo. Our seat is almost on the floor. You can take at least 5-6" out by getting rid of the seat track. We use 2x4's and steel srtips..but we are all about the same height so we don't need to move the seat between drivers.
I really hope the 2x4s are not a structural part of your seat mounting...
And inside available height is different per car and set of drivers. I'm 5'7", and my brother is 6'1", but seated he's only 1.5" taller than me, his height is all his legs. My 6' tall brother in law however is mostly torso and much taller when seated. In our old Daytona we had the seat on the damn floor with the cage professionally tucked to the roof, and we were all super close to the limit. Not all cars are the same.
Cages outside are common, and allowed. Just pay attention to angles of other bars coming off it. Run the plans past HQ before you execute so they can sign off on it or point out areas to change. The advantage to outside cages is that you don't need to mount your seat in the depths of hell, which often limits your vision out of the car.
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