Bearings Main Set STD (Genuine) STC3724

You can get away with using the cheaper main bearing shells: RTC1718 one of the shells locating
notches line up with the notch in the block, but the shell that fits the cap does not line up with their notches,
so, file down the notch on the bearing shell and the shell will then fit the cap.

The other shell still has its notch which will prevent the shells slipping.
If you're still concerned, a drop of green loctite on the back of the shell (the one that has been filed)
will definitely keep it in place)