Helping JimT out with an inframe ...

[Chamberpains]

Patience is your friend JMBT. Your way is fine with a 1/2" breaker bar. Just get it to move a very small amount and work it back and forth and they come right out. It'll be even be easier to do if you followed Cummins procedure and put a light coating of antisieze on them when you put them in.

Also, if your overhauling the engine, you're gonna throw the head and the old bolts away. Why waste time, gun them out. If they snap, who cares? It all goes to the scrap pile, right?