You will be very fortunate if your late '69-74 front bow plates are in good, re-usable condition!
The new plates are made to install just like the original ones. Careful work is required to remove the main bolts and the old deteriorated plates. After a careful cleaning of the front corners of the frame the new plate or plate can be readily installed.
With original or with replacement plates the proper mounting and orientation of the header bow also takes time. Adjustment of the "strike angle" is very important. This involves "shimming" between the bow and the plate so that the curve of the bow is as parallel to the windshield as you can get it. Gluing a weatherproof seal to the lip of the bow, and
trimming the seal will also help.
The original bows that we have seen were adjusted on their frames at the factory in similar fashion.
We urge the use of wood, not "wood replacement" plastic bows.