The torque spec is greatly influenced by the coefficient of friction between the threads. If the spec was 75 lb-ft of torque dry, and you used 30W oil on the threads, then it is likely the torque was too high for the bolt. Or, it could have been a weak bolt. In those cases, you can usually feel the bolt stretch (it gets measureably thinner in the shank region) and will not hit the desired torque. I had this happen to me on the rocker shaft hold down bolts on a 340.
You probably will need to pull the head to get the broken bolt. In my experience, a bolt that broke during torqueing will usually just back out of the threads if you can get ahold of the remaining part of the bolt.
