The bell housing locating dowels on my 97 2.0 Zetec are 15.5mm in dia, the corresponding holes in my MT75, (removed from a 92 Sierra twin cam) are 12mm dia. Does anybody know if I can fit smaller dowels or will I have to modify the bell housing?

since the dowels line up the box n block you are better off drilling the box to 15.5mm or making stepped dowels

I agree with drilling to fit larger dowels