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Chris,
That groove is wear-induced, but it should not be on the crankshaft itself, rather on the shank of the crankshaft pulley which encases the front of the crankshaft. The original fabric/asbestos rope-type seals were pretty tough and eventually wore down the cast iron of the pulley shank. You can obtain a new pulley from Snyder's who offer a two-piece semi-modern version where the pulley and shank are separate. Personally, I prefer the original one-piece version, which aren't that difficult to find.
The only seal in the rear is a cork one between the curved lip of the oil pan and the outer groove in the rear main bearing cap. Within the rear end of the main bearing cap on the crankshaft is a circular slinger which causes the oil trying to escape to flow out the tube in the front of that bearing cap back into the oil pan. A modern seal can be incorporated, but some machining is required, I believe.
t