Do I need to keep that flex hose that connected the brake line to the tee fitting or can I go directly to the tee fitting with the nickle/copper tubing
Typically the rubber flex lines are used where there is going to be movement.
Like between the body and the rear axle and to each of the front brakes due to turning flexibility needed.
I'm not sure how you would plan to go from the body to the axle with hard lines