these studs are extremely hard and especially brittle.i suppose that its the heat that they have to endure.even when they are new they can shear off very easily whilst tightening unless you release any other pressures on the flanges i.e. a hanging exhaust your trying to mate up. rimmers dont sell these now for the kv6 ... only when buying a new manifold complete. i had the same issue as the op has. the broken stud would not tap out or drive out in situ too much flex.... they are on splines and are driven in sometimes not aligned properly when the bolt is first tightened from new. he is correct .. no room to manoever very tight in there especially lying on your back working. i did however manage to remove 2 of them using a ball joint separator with a little improvision. would have been better with a bolt and nuts setup but there must have been a reason for this set up ...