A total of 7500 safe deposit boxes were holsted out of the basement of the Mississippi Valley Trust Company yesterday and moved to the Mercantile Commerce Bank & Trust Company, Eighth and Locust streets, where the new Mercantile Trust Company, a merger of the two banks, will open Tuesday. Armed police and bank guards surround one load of the boxes just after a portable crane had hoisted them from the Mississippi Valley basement through a hole cut in the sidewalk on Broadway at Olive street.