You are right about the 2 for the heat shield, My 68 has a/c and from what I can tell has never been apart. My motor has only one other studded head bolt. It is the second bolt in on the passenger side and is one of the bolts holding on the main a/c bracket and the stud is used for the negative battery cable. As far as a fourth mine does not have one. I know motors slated for different body styles and years had different setups from the motors I have disassembled through the years are you dealing with a 68 firebird motor and bolts? The only other thing I can think of is the motors that came with the rocker baffles or as some people call them oil drippers I think they were held on with studded head bolts.