Your rescue papers should state the supposed breed mix and might be a last recourse that would help you out in the event, God Forbid, someone took issue with your dog. Boxer x Lab is not an unusual mix.
But sadly even dogs with absolutely no pit type in them have run into trouble. At few years ago near me Police shot a confused and upset doggy who had escaped her fenced yard. She was a chow mix and after photos were published she looked more chow than anything else. But someone said she was a pit mix.
With your plans you would have to hold yourself and your dog up to exemplary standards. You'd want to make yourself known in order to attract potential owners for your rescues, advertise your intended boarding business. You and your dog would soon be known and someone might not like it, let alone your dog. We live in a sad, sad world sometimes.