I had heard or read that the 273/318 block is weak around the drivers side motor mount ears and that the 340 360 has reinforcement in the block there, this is why the left motor mount brackets dont direct swap between 273/318 and 340/360. Im not 100% sure about that and I'm sure someone would say I'm wrong especially the "318 is a 340 " guys...either way it is something to read up on IMO. I seem to recall that it has to do with the crank journals and it is obvious the main caps are beefier in the 340 360. The 318 wasnt designed to rev as high as the 340. The guys that say a 318 is a 340 sort of miss the mark with that statement, Chrysler designed and cast a different block for a reason when they designed the 340.