The process of installing a rear seal starts with some simple machining of the engine rear plate. Please ignore the oil spots on the backdrop paper, as there will be some of that in many of the following pictures. To allow space for the new seal you must enlarge the big hole in the center of the engine rear plate for more clearance around the crankshaft flange. And while you're at it, add four tapped holes for screws to hold the new seal in place.
The drawing below the pictures gives the dimensions for machining. Notice that the enlarged center hole cuts into the counterbores around three of the original bolt holes.