All good points by Neelam. To summarize - you need to understand that a fired heater is really just an extension of your piping system from an analysis standpoint. It's just that that section of the piping system is encased in a firebox.

Typically, the nozzle penetrations through the box are not fixed points. I've had bottom outlets heaters with the lines supported at the top of the box, resulting is 6" of downward movement at the flange face. The external piping system that you design needs to account for whatever the heater piping is going to do.

Also, the heater shell is going to have a skin temperature that will need to be accounted for - I've typically seen the refractory thickness set so that the skin temperature is about 200°F.