when can you use TTI headers after the engine is built?

Just during the first 20 min of the cam break in when there is a lot of heat.
Some people put on an old set of headers or manifolds for cam break in then put the TTI's on. Others have done it with the TTI's on and just used a fan to help cool it. Another thing is if the carb is running lean it will cause excessive heat so they try tunning the cab a little rich.