I think all Burstbuckers are a "standard" 49mm spacing. Then again, loads of Gibson guitars have 500T or 498T bridge pickups, which are 52mm.
My point being, don't measure the pickup, measure the strings - directly above the bridge pickup polepieces. Then get whichever is the closest match. 50mm is usually fine, but sometimes the tune-o-matic saddles are notched to give a wider spacing (depending on who set it up at the factory), so 53mm can match just as well.