If you have an acetylene torch (doesnt everyone
) it will provide a ton of heat to break the rusted bolt loose in short order. I do that all the time on rusted bolts. The way the bolt is positioned youll never get enough PB to wick up on the threads.
Lacking that, a propane blow torch will work and better with Mapp gas which burns hotter (yellow gas cylinder). Concentrate the flame on the head side of the blind hole on the bump. This will transfer heat to the bolt to break it further loose. It might take a while as it is cast iron. NOTE
: Although the flame is concentrated, take the usual precautions; clear away all wiring, make sure no fuel leaks, wipe away the PB Blast, and have fire extinguisher on hand--every car guy garage should have one.
Depending on what tools you have, another alternative is to use a cold chisel and/or a die grinder with a carbide tip and remove the bolt head, lower the header and use a vise grip to get the threaded portion out.
As for replacement header bolts check the various suppliers like Jegs and Summit and see what type of bolt heads are available, or just use an appropriate size bolt with a head that clears the header tube and provides wrench contact. The torx as you suggested or maybe an allen head cap bolt. As Goat Roper stated install the new bolts with an anti-seize compound.