I would add that before you add that 70A breaker to your existing panel you need to look at the amp rating on that panel and see what you're already running with your existing circuits. Unless the house was custom and the owner specified the service you may only have a single 100A service (or less) into the dwelling, and you would not want to sub-feed 70 amps out to another box if that's the case. You may need to run another service into the house for your increased 240V loads. Just because you have a breaker space remaining does not mean you have feeder capacity for your new load.