If the seal surface on the crankshaft is not near perfect, then a neoprene seal will not last very long.  In these cases, a rope or Teflon seal would be recommended. I'll add that if putting a neoprene rear main seal in a 312, make sure it's for a 312 and not a 292.  You're dealing with two different seal sizes in this case.

Here’s a link to a past thread showing a picture of a pitted seal surface.

http://www.y-blocksforever.com/forums/Topic61335-3-3.aspx 

Lorena, Texas (South of Waco)