What rpm stall converter should use?

How does that work on the freeway?
The car has a 4.56 axle so that limits any freeway use. But on semi rural roads at 45-50 mph, no issues as the cooler works just fine (mine bypasses the rad and goes to its own cooler). Driving away from a stoplight in traffic, you'd never know it's a slippery conv. But if you lean on it, hang on!!