As other have said, the torque converter is matched to the engine/cam combo. The idea of added stall speed is to get the engine into the torque quicker since the converter lets the motor rev higher before it “locks up”. Too much stall speed can generate serious heat though. Make sure you pick a something that is below your general operating rpm when driving.
The smaller cooler is for the motor, larger is for the tans, I've got another cooler about the same size as that trans cooler that I'm going to use and double it up with.