What are the best exercises for ground-and-pound stiking power? I know technique is important, but are there any conditioning exercises that will help develop power?

You're correct that ground and pound power (like all strikes) comes from good technique. On the ground, this means strong positioning. Getting into a strong position, such as a good top mount, is very technical. But, at the same time, there are several good conditioning drills that will help you practice getting into various positions quickly and maintaining them. The wrestler's spinning drill (p244) is great for developing ground mobility. The same exercise carries over into top and side mount control, not just back control. And you can practice it with a partner or solo using a heavy bag on the floor.

On top of this, core strength is where you'll generate your striking power, especially rotational core strength. Try all the different sit-up variations for general core strength (p159), and then the isometric hip throw (p171) and medicine ball twist pass (p177) for rotational strength.

Since groundwork is really a full-body activity, every aspect of conditioning that you work on will improve your ground game. A strong back, biceps, and forearms will help you hold and move your opponent. A strong neck will allow you to use your head as another limb to control him. Strong legs will help you drive forward to keep your opponent on the ground. So, to summarize, so long as you focus on developing great technique, any conditioning you do will only help improve your game.