I tried to automate the wither building with a builder. It works basically.

The only issue is the placing of wither skulls. It works fine but the skulls are placed 2 blocks below where they should be. It did the same with zombie heads (i tried them for testing).

I worked around this issue by placing the skulls 2 blocks higher in my space chamber and it works like a charm.

Best regards
Kisox

This is probably not a bug but just how things work. Can you show the setup?

This is my setup to spawn the wither with the builder

http://imgur.com/a/s0WYh

Still experiencing this in 1.12 in FTB Revelations 2.2.0 beta. I don't have the mod version number on-hand at the moment.

In my case it's actually beneficial because it allows me to define the skull space on the same plane as the soul-sand space, and have the builders stacked with crates in between for material storage, but It is definitely a bug given the block preview shows the placement as being two blocks above.

Having a similar issue with FTB Eternal 1.12
1.12-7.73
Just spent 20 hours building a fully decorated wither farming building before testing my mechanism (dumb), and was worried placing skulls was deliberately disallowed. But after reading this and experimenting this is the behavior I can tell for placing a wither skull. For simplicity lets say you are trying to place 1 skull with the builder:

• It will not place into the block you set as the destination
• If the block you set as the destination is filled, it will not place a block.
• If the destination is not filled, and there is space under the destination, it will place the skull on the first surface down from the destination.