Increase Wither Spawn
The mod is specifically designed for Minecraft version 1.7.10 and doubles the spawn rate of wither skeletons in the Nether fortress. Currently, there are no configuration options available, but they are planned for implementation in the future version 2.0.
This modification might be particularly useful when used alongside "IncreaseMobs" due to a bug in that mod that completely prevents wither skeletons from spawning. Combining it with our solution restores normal spawning of these hostile mobs.
Version 1.1 is fully compatible with fortresses from Magic Cookies! The number of wither skeletons in them increases significantly.

Just enable the mod - and you're done!