I use wood blocks. It spreads the contact point of the jack or jackstands over a larger area. But 99% of the time when I'm raising or lowering a car, I'm using a lift. I've given up on doing big jobs with driveway tools. I always break something.

Don't worry about your sub-frame. You can't see it when the car's on the ground, and you'd be surprised what most people's look like. Many garages don't even know how to raise and lower a car properly, so most older cars are pretty beat up under there. It won't affect how it drives.

Look further in away from the body, and you may see another frame reinforcement on the underbody. The outer points where you use a service jack aren't actually parts of the frame on most cars. They're made for your service jack, not a floor jack.