Use the "object" Item in "Insert" Menu Item. Make sure that you click the check box "Link to File"

This way when you modify the main header all your files will be updated.

NOTE: You may need to right click on the link to update the link. Though if the 150 files that you use are closed when you modify the main letter head template then they should be updated automatically.

ScreenHunter_01 Nov. 25 17.06.jpg

ScreenHunter_02 Nov. 25 17.09.jpg

Paul D