Having done at least a dozen of theses cylinder heads if the piston liners are allowed to move even the slightest while the head is off then little bits of cooling system scale will fall down between the liner and the block causing the surface to not have a good seal. Coolant will end up in the oil. Whenever we pulled those heads we would bolt on a special home made clamping device to hold the liners down and not allow them to rise up even the smallest amount. If those liners have moved you will need to pull the oil pan and remove the pistons and liners, the liner seals have tabs on them. Make sure you replace the liner seals with the correct number of tabs. I think there are three or four different thicknesses with different part numbers. You will not need to measure the liner height as long as you put the correct liner back into the correct hole with the correct liner seal. Sorry for the long winded response.