Most of the nipple wrenches on the market are way too soft. Many shooters are forced to make their own from better steel.
After trying Fingers McGee's suggestions, and if after all that the nipples still won't budge, you will be forced to drill them out. Don't do that with a standard drill and drill press, if you are at that last resort, after finding the thread involved, use an endmill or carbide drill with the cylinder locked in a mill vise or index head to assure you don't damage the threads.
Do note, however, that a "58" Remington nipple is mounted at an angle, unlike the Colt which is parallel to the bore. If you are forced to machine it out, you will have to match the angle.