You are better off to switch the AC side, relays that switch very high DC currents are designed with contact arc quench methods, such as made by P&B, which use magnet arc blow out.
An ordinary relay can look like a Plasma torch when breaking high DC loads, which ends up either melting the contacts or fusing them together.
You could use the braking resistor in series for a certain time or until the current drops, a better method might be a simple Triac circuit with ramp up in the AC side.
Although if using the supply to feed a Viper drive, it may be time to look for a method of its own supply!.
Nosmo.