Its pretty cut and dried.......

If you have MW size stage VI heads on your RB block, the 128 intake fits that combination of parts.
And nothing else.

The 337 intake is for MW sized ports with heads that don't have raised ports......like factory MW heads, or Edelbrock RPM heads that have been opened up to MW size.
It eliminates the need for a separate valley pan gasket, since it seals off the valley with a plate cast onto the bottom.
It can be used on some raised port heads as well(Indy, Victor, TF270, etc), but there will be a gap between the block end rails and the built in valley plate.