The "original" header bolts were unique to the Pantera. They are similar to what you can buy today from sources like Mr.Gasket as header bolts.
They are hardened hex head 3/8"-16 NC threaded bolts BUT the heads were unique in that they were not 3/8" hex heads, they were m10 metric.
They don't match up length wise to what is currently available either such as 3/4" long or 1" long. I can't tell you the exact length since I no longer have any here, but they may have been metric in length as well? 23mm seems to come to mind but I may be incorrect on that?
More then a few early owners were stumped when trying to remove their original DT supplied headers when the sockets would spin on the heads of the bolts.
I don't know of any current sources of that combination now.