In machining work you TRY to do everything with a single set-up of the workpiece in a machine, Dan. In working on a crank, the concentricity of the main bearing surfaces to the rear seal diameter are important.
It is realistic to move the the crank from the crank grinder to an engine lathe and using a a four jaw independent chuck and a dial indicator, accurately set-up the crank to permit turning the rear seal area so that it is concentric with the previously ground main bearing surfaces.
By my calculation, 0.130 inch needs to be removed (in total) to accomplish the needed modification to the existing 312 seal diameter. The reduction of the slinger diameter by 0.200 inch will likely be done at the same time of the set-up to cut the rear seal diameter.
While the '55 cranks did not have a rear slinger, I like this lathe turning alternative to grinding away the rear slinger which some shops do because of not having the appropriate width crank grinder stones.