The divider wall is the wall betwee ports. It's usually straightest of the two port walls. In most cases you only have to worry about the head bolt hole protrusion.
I have used JB Weld (original not quik-set) to repair heads and blocks with good results. The block I'm running now was cracked between the lifter valley and water circulation opening in the deck. I drilled through the block at the end of the crack and ground a gruve along the crack from outside to inside the block. I filled it with JB Weld and let it cure with a 100 watt bulb in a reflective shield fixture. That was in the mid 80's.
Ray