How to change the lava filling level of caves

Hello everyone!
 I have a datapack that changes the height of the world (min height - 304, max - 1920). 
But my problem is that all caves from -54 altitude and below are built with lava. I want to change the level of this lava so that it starts filling the caves, I don’t know, let’s say from -304 height.
So, yes, I really need help. Thank you in advance!