The converter from the ST300 GS will have a higher stall speed, I'd use that one. Should be about 2000/3000 rpm. The Riv converter is larger and will have lower stall speeds. The 401 GS flexplate should have two sets of bolt holes to fit either converter. The flex plate you pictured will only fit the larger converter. The converter you pictured does look like the smaller 12" one.
