I never nailed down an answer for that. The closet I ever heard was Chrysler called out the thrust main on the 340 bearings only have a half groove for some reason I have long forgotten.
If you get the bearings called out for a 318 all the mains will be grooved. Why that is I haven’t the foggiest notion.
Now that I type that out I may have that backwards. It might be the 340 bearings with a full groove thrust. Either way that’s how I remember it.
Even thought the dimensions are the same they call out different part numbers for 318 and 340 bearings.