Where can I get engineering info drafted for a custom header?
The city wants design documentation stamped by a licensed Professional Engineer.  For off-the-shelf components the supplier may be able to provide a generic stamped design and as long as it's installed and assembled according to the plans, it will be acceptable.
But it you are building a structure with lumber or other piecemeal components, the design itself needs to be stamped.  No &quot;special software&quot; will cut it.  It needs to be an engineer licensed in your state.
For what you appear to be doing, basically building up a structural member from components, it will need to be analyzed and stamped before it will pass inspection.