Hi mate
Ok, I hope I am understanding you properly. Not sure what you mean by the angle brackets holding the sign together but here is my explanation for doing signs on factories at this kind of length
1st take 2 lengths of inch-by-inch angle. They should be about 16 ft each & drill holes along one side of each.
2nd using a spirit level and cordless screw gun using tech bolts.. (Starting from centre) screw the angle to the wall as shown.
3 repeat again with second length of trim using level as you go along.
What you should have now is a nice strong ledge 100% level. Very easy to fit because it is so light.
4 check it is level all along without any sagging areas.
5 starting from the centre again using the order indicated for the panels.
Sit the sign parts onto the ledge. The ledge will allow you to easily lift the sign onto it freeing your hands while someone else holds it against the building. The angle ledge will take all the weight and keep it perfectly straight and free from slipping while you get it fastened.
6 repeat each section as before.
7 once all panels are secure and checked. Remove the angle ledge from below 7 take it home.
The only side effect from this method is that you have several small drill holes were the angle was. But from the ground you cant see them and they are so close to the sign they are not noticed.. If it is high up, youโre laughing