I need to replace my basement window but I am not quite sure what measurement to use for it given that the current frame looks like it is actually embedded into the concrete. The situation is also complicated because the concrete has a slope on all four of the outside edges. See image for details. The concrete is 31"x13.5". Do I gut out the opening and just add wood to level it out to the concrete, then get a custom made window of 31 x 13.5? Or, HD has 30"x13" windows, can I make a wooden frame that protrudes into the window opening slightly and use the HD window? I realize there are multiple posts on this topic but my situation seems to be slightly different from everything I have read.