How to make Minecraft non-SF world deeper?

I have this idea for an underground castle in a massive cavern, but I have never found any area with enough ground depth to make a cavern with more clearance than about 80 blocks without ending up with holes in the ceiling, even under mountains and going down to bedrock. Is there any way to make the world deeper with the advanced customization settings that doesn't also make everything stretch out of proportion or something?