Measuring distance between overlapped images using their corresponding datasets

Hi, I have two (of seven) 8-bit datasets of the same object growing with time. I would like help determining the distance between the fronts of the datasets (essentially measuring the object growth). The voxel size is 1.3 micrometers and with the distance I would like to plot the average distance of growth against time. The datasets are at times 1 and 2. I have added an image of the overlapped images for reference. Your help will be appreciated. Thank you.

Data: https://www.dropbox.com/s/3kdlze4cyzxt673/Data.zip?dl=0