No guarantees it won't effect the strength of the sole but I would do it if I was in your situation as I think it's probably safe and the shoes aren't worth much to you since you can't get the cleat position right as they are. I would just extend the slot but would want to be very precise about getting the same width of the groove and making it perfectly straight. I'm worried that if the groove is even slightly too wide any bolt anchor could eventually pull through the sole. All just speculation of course but experience is my guide.