Cast iron properties

I need to carry out a repair on my Boxford shaping machine vice. The original has a two small sections of the movable jaws fractured off. I intent do fabricate the whole movable jaw, which will require new dove tails to be machined.

A length of 60mm square cast iron would be the starting point but I'm unsure which grade to purchase M-Machine do GR17 and SG. Which grade be more suitable ?

GR17 is your basic cast iron with flakes of graphite that make it softish, easy but messy to machine, break if you hit it hard but cheap. All those flakes or graphite are stress raisers which make it more breakable under bending or stretching but don't matter when squashed.
SG is Spheroidal Graphite. It is treated so the graphite clumps together as globules which reduces the stress raising function of the flakes so it is much stronger. It is what is used for brake discs and exhausts to be cheap and strong -ish.
In both types the base material or matrix that isn't carbon in one form or another is like mild steel. Have you considered using mild steel? They only used cast iron 'cos it was cheap in the quantity they were making and got close to shape reducing machining. Nowadays it would be CNCed from billet.

The SG iron has two advantages, the much lower brittleness which means your replacement will be superior to the original, and the very much nicer swarf which is more like steel swarf.