I wouldn't tap it bigger if I were you, it's a right pain in the arse. You need bigger bolts, you need to file the clamps out, you need bigger booster spacers, file the booster out. Just get it helicoiled, it's so much easier. I'd also make sure you have bolts that are as long as possible. Get ones that are too long, then cut them down until they fit, I've stripped mounts before from having bolts too short, you need at least 6/7mm of thread inside the frame, I've currently got about 12mm.