There are clamp on brakes, but it is probably best to have the muzzle properly threaded on a lathe.
I wouldn't even consider trying to thread the muzzle with a die.
The procedure I use is a common one... Set the barreled receiver up in the lathe, running it through the headstock. Indicate it in at both ends, the muzzle end being indicated in on the bore. Turn and thread the muzzle and screw on the brake. Bore the brake to suit the caliber - perhaps 20 thou over bullet diameter. Doing it this way guarantees that the axes of the bore and brake are exactly aligned. The brake may be indexed by facing either the barrel shoulder or the brake. Brake is torqued into place, no peel or crush washers.
Another method is to set the barreled receiver up with the muzzle on a live tail center. Turn and thread. Install a brake with a finished bore. Touch up the crown if needed, if contacting the live center has raised a burr. This is not as precise a method as the first one. Faster, therefore cheaper, probably acceptable results.
