You can ask the vendor to provide you with the terminal point displacements .Else you can model the stack as a pipe ( with the pipe temperature as the profile for different operating conditions)and check for the loads are the flange to see if they meet the requirements.Some consultancies also follow an OPERATING STRESS CHECK ( In CAESAR II Terminology W+P+T Stress check) and limit the value to 3000 psi at the interface point.This 3000 psi I believe is a modification of the otherwise popular 6000 psi check.

Regards