It says not to be used with back plate. That's kind of strange because look at the front of the timing cover. Would the water flow ok with all the extra space?I guess cougar folks don't get to much exposure to pre-67 year model engines. Check out this link and you will find that the top left photo matches the picture of the OPs pump. The site has so many pumps.....
The correct timing cover is the back half of the pump volute. Note the drain slot at 6:00 OC on the picture you posted. I will snap a pic of the correct timing cover.