I've done them on a couple of rifles. I assume you paid close attention before you started grinding and made sure you're grinding on the slightly discolored spot where the top of the pin was welded over and reblued.
Grinding too far is certainly a real problem. One thing to remember is that even if you slightly grind into the threads, twisting the brake off is going to un-gall the threads unless you really dig in too far. You can actually tell when you get close to the threads. If you slow down, you'll just barely grind into the top of the threads in a small area. The real trick is finsding the pin. Grinding usually blends the pin in perfectly with the surrounding metal. When you get close, the key os to use a wrench or a screwdriver and wiggle the brake back-and-forth. This will usually reveal the pin easily. It may take considerable torque to break the pin loose. Once you do though, it usually falls right out and you're home free.
Try to finish it yourself - it really isn't rocket science...