@Alan Merklin is probably a better guy to hear from, but in the 4 or 5 CMC windshield header bows I have dealt with, NONE of them fit right out of the box. ALL of them had to be trimmed or relieved or re-shaped in the slot for the windshield, sometimes a lot, to make them fit the windshield top frame properly. They all had the basic shape, but just didn’t quite fit here or there.
They’re supposedly made from fiberglass but it’s the weirdest damn FG I’ve ever seen. The good news is that it is easily re-shaped with a Dremel tool and a 3/16” hasp/grinder. Once you get the slot shaped so it fits down onto the windshield top frame OK, then you can use some Frost King 3/4” weather strip (Home Depot insulation aisle) to put one or two layers up into the header slot to seal against the top windshield frame and make it rain proof.
You can glue or re-glue the top material to the header with DAP contact cement (again, Home Depot Paint dept.) Just brush it onto the header and the top material, let it set up and then glue them together. I think my old article on how to install a new top is still up under the “Resources” tab on this site. That gives detailed instruction for all of this.
Having typed all this, I believe that Alan throws the CMC headers away and uses headers from Vintage Motorcars in California (because they fit better). Don’t know if he uses the DAP cement or something else/better.