Cape, there is a good chance they will not know what length pin was used unless it is a Cav Arms upper. They sell many of the Cav Comp brakes as separate items, that's how I bought mine. The gunsmith I use installed them as a service, and they provided the brake. The best way to remove it does not require knowledge of the pin length, but does destroy the brake (to save the barrel threads). I usually grind down to the pin head, then mill around the pin until there is enough of it exposed to pull it out with clamp pliers. Some gunsmiths (like mine) use a threaded pin or machine screw (with head ground off & welded over) so you might have to create a slot to unscrew it, or clamp and rotate it out - never force it or you could damage the threads. If you try to drill it out, you risk penetrating the bore (not good), or damaging the threads since it will be hard to control the depth. I would alway rather sacrifice a $25 brake than a $200 barrel.
Anyway, good luck!