Headers or Manifolds for a 67 4spd Cuda

Guys I am a bit confused. I was reading the installation instructions from the TTI website and it reads for installing a set of headers to remove the torsion bars first. I am not understanding why I would need to remove the torsion bars? Also if its true to install the headers on a car with the motor and tranny in it that removing the torsion bars is neccasary what if (like in my situation) I am installing the motor and possibly the tranny at the same time. Can I lay and then tie the headers in the engine bay in their location before installing the motor? Sorry for all the questions but this is my first time messing with a muscle car, LOL. I come from a truck family. Thanks for all the helpful info:)