I bought a set of Stage 8 locking header bolts made for BBC (longer shaft than Pontiac). On the same bolt you are having trouble with I got a couple of parts store 7/16 head header bolts, ground the top corners of the hex round for more clearance and drilled two 1/8" holes across two of the flats. Then I used an 1/8" allen wrench or drill bit to tighten the bolt. After they were tight I put a cotter pin in one of the holes and bent it so it just touched the header pipe and will not come lose.
"F'n" stuff up since 1990!
Some are engineers or fabricators for a living, I do it as a hobby.