The two manifolds on the L24 engine are shown in the parts diagram to be secured with studs at all seventeen locations.  However, the engine in my 70 Z uses bolts for the top row (six locations).  A spare L24 engine (from a 72 Z that I had as my daily driver back in the 1970's) also used bolts for the top row.  I'm 90% certain that the head and manifolds on that car had never been removed for service (I was the 2nd owner and bought it in 1976 with only ~ 40,000 miles on the odometer).  The bolts