Ive done something similar in my world, its pretty easy if you look at the wikis advanced worldgen settings.
Cavern layer is the easiest to set, find this line:
[CAVERN_LAYER_COUNT:3] and change it to [CAVERN_LAYER_COUNT:1]
You now have only one cavern layer. Be warned that you don't always get every single type of cavern tree as a result (there are no nethercaps in my world, I do get bloodthorn though).
To have solid Z levels, find:
[LEVELS_ABOVE_LAYER_1:<number>] - its probably set to about 5, so set it to: [LEVELS_ABOVE_LAYER_1:25]
This will give you roughly 20-30 z levels of stone+dirt above the first cavern layer for you to build in (it can vary quite a bit depending on location).
If you don't want your candy supplies nerfed, you can increase z levels for [LEVELS_ABOVE_LAYER_4:<number>] and [LEVELS_ABOVE_LAYER_5:<number>] - its not an exact thing but generally if you increase these a bit you make candy spires higher.
To get 80 z levels beneath your cavern and above the magma sea, its probably [LEVELS_ABOVE_LAYER_4:<number>] you need to increase.
The wiki explains it all quite well:
http://dwarffortresswiki.org/index.php/DF2014:Advanced_world_generation#Cavern_Layer_Number