Actually (just a second while I finish this jelly donut) I was looking into the feasibility of putting a 618 head on a Mk2 bed. There are two issues with the Mk2 design: first, the spindle deflects easily due to the close bearing spacing. Secondly, the driving force from the main step pulley is coupled to the spindle through a single small woodruff key and zamak coupler which is a high wear point. The 618 is better in both respects, and would fit providing two more holes were drilled in the bed; I'm not clear about the leadscrew and tumbler fitments however cuz the 618 spindle sticks out so far to the left compared to the Mk2.
Are you sure it's 1.25"? The pictures I've seen sure look closer to 1.5" (the Mk2 is 1.25")
Mark S.