While waiting for more expert responses, you might try playing with the crop scissor tool thingy. 8x10 and 5x7, being different aspects, might result in the 5x7 cropping when/where you don't want.
Don't ask what made me think of this as a possible solution.
--George
George hit the nail on the head.
The only thing I am adding is a screen snap of where the autocrop button is located.
See my white cursor on the button at the bottom....
Fred