I have a poured 8" thick concrete foundation in basement that I need to replace an entry door. I would like to use a prehung door but the height of the entry is approx 81 3/4". The door I was looking at needs 81 3/4" so im guessing it will be tight. I dont want to special order a smaller door or get into cutting standard door down. What would be the easiest method to shave down that small amount from the concrete? I planned to install 2x buck on the sides for the door to attach to. Is there any issue not having 2x on top of the door? I never installed a prehung door before but I am assuming it doesnt get attached at the top anyway. Thank you.

We recently moved into a new home. The front porch appeared to have muddy tracks on it which we assumed could be easily washed off with detergent and water. After unsuccessfully attempting to clean it, I learned from the worker who poured the concrete that workers had walked on the fresh concrete with muddy boots on. What is the best way to remove these stains without causing any pitting or other problems?