M8 seems a bit slender for a cross slide feedscrew, John, particularly when it will be used in compression unless you are boring a hole or facing from the middle outwards. Even 7x mini-lathes use M10x1, which has 75% more the cross-sectional area on its core diameter.
If a replacement is to be made and space permits, I'd seriously consider an M10 replacement screw and a nut to go with it, with 1mm pitch for one turn=1mm, and a 50 division dial.
Did you bolt your new bracket on firmly before fitting the cross slide? If so, that might be the cause of the screw breaking. If there was any axial misalignment between the nut and the bushing in the bracket, the screw would flex slightly at first to accommodate this, but as the nut got near to the bracket, the strain may have become intolerable. Better to leave the bracket a bit loose until the cross slide has been wound back as far as it will come towards you, so the bracket can float into the right position, then tighten up the fixing bolts.
Andy