It is not necessary to remove the windshields when replacing the front cap. If the windshield gasket "lock" is removed from the top of the windshields, the gasket will fold down enough to allow removal of the rivets.
The labor time depends on the method you wish to use.
If you add another seam in front of the roof and above the driver and passenger window, the "overlay" time is in the order of about 8 hours.
If you wish to make the replacement look "exactly" as built, then the cap must be removed. You will then also have to remove the driver and passenger windows. The time now will be 20 to 25 hours. Getting the new cap under the roof and window sheets, is no easy task, and must be done very carefully.