Tough call! My gramps would totally have known what to do, I'm just a dilettante.

I'm worried about the tree dying back later from the exposed cut, however you prune. I fear the correct thing to do would be to cut off both sides of the split and put a thick layer of bitumen over the cut area. Leaving any branches above the split risks the split opening up again under their weight next season :(

Hopefully someone with professional experience chimes in!