Superdave,
Welcome to the forum. It's great to have another Phantom build

As peter-curacao mentioned, on your strongback, you may want to add some lower 2X4's lengthwise, connecting your legs near the casters. As your boat gets heavier, there will be a lot of leverage on the strongback legs, as you roll it in and out of the garage. Any movement of your strongback legs may cause your boat to not be level.

The more you reinforce your strongback now, the less potential problems you will have later.
-Mark