Idle is not going to change your compression/vacuum ratio. A leak is a leak regardless of RMPs. Your head gasket is usually a "special" gasket and usually fairly indestructible. It often needs to be a specific material to transfer the heat from the cylinder to the head properly. Put if back in if its not destroyed. The base gasket needs to be replaced EVERY time you pull the cylinder. Also, the mating surfaces need to be thoroughly cleaned. The base gasket often leaves pieces behind. They need to be cleaned off without scratching the mating surfaces. Also, make sure to torque the cylinder and head back down to the manufacturer's recommended ft/lbs.