Are you shure the head has a crack?  It is some what common for leaks to
develop under the bolt heads.  Thes can be decieving.  They can also be
easily fixed fixed by purchasing new bolts, using sealant, and installing to
specs.  Fel-pro "Bolt=Prep" is great for this application.  It acts as both a
sealant and lubricant for the threads.
Dennis.