Thank you sir.
I just read his site and this is his reasons why he made a bolt on cage http://underwood-racing.com/roll_cage.htm
"The entire unit will be "bolt-on" for a variety of reasons. 1) It will be easier to finish and panel the car with the roll cage off. 2) There will be times when it will be handy to have it removed for maintenance on the car. 3) If the car ever gets wrecked, and the cage is still good and the frame is tweaked, it could be used on another frame."
His car is awesome! I also torn between a bolt on cage and a cage integrated into the frame.
A plus for bolt in is that you could make two cages. One hardcore cage for the track and one for driving on the streets.
A plus for integrated cage is its safer and stiffer.