First of all, congrats on the free motor. I am using a 5hp Gamefisher on a 14 foot john boat. It's decades old and only gets used a handful of times per year but works like a charm whenever we take it out. Your 7.5hp motor shouldn't be too strong for your 10-ft john boat. My 14-foot is rated to accept a 15hp motor. I think you'll be fine.