Minecraft stores all blockstates in a single class, meaning their logic is the same no matter of the id or damage values. For example, all command blocks are in the BlockCommandBlock class.

EDIT: Just did it, the code is in BlockDynamicLiquid.java, you need to find if (this.canFlowInto(worldIn, pos.down(), iblockstate1)) and change all instances of pos.down() inside the {} brackets after if(...) to pos.up()