Problem with Cam install

I am rebuilding a 1999 V6 Mustang and I installed a Comp Cam 480 cam in place of the stock cam and new timing chain and gears. I had installed the stock camshaft sensor gear and torqued it to the required specs and then the engine would not turn. It makes sense since the the end of the camshaft is not flush with the end of the drive gear and when I set the torque it would pull the camshaft agains the thrust bearing. What am I missing to keep some play in the camshaft?