Wait - you mentioned you had both soul shards and MFR installed?
Wouldn't it have been easier to set up a basic MFR finder hooked up to either a zombie soul shard, or just a regular zombie spawned found in a dungeon, and then use that mob essessence in the mob spawned thing from MFR with a witch in a Safari net? One could even get the original witch from the twilight forest by transformation powdering a Druid. You could then drop the witches a certain number of blocks (enough to one shot them after the fall, but not kill them) and stand tapping the click key at the bottom with the soul shard in your inv. Doesn't seem chesty like the anvil, due to requiring power etc, but a simple build non-the-less.
Nope, I use the DW20 pack, which doesn't have MFR. My mentioning it was just an acknowledgement of the fact that there's several options for when you want to set up a mob farm, and so the block I was suggesting wouldn't be amazingly useful, especially post-early game, as those other options allow for more efficient and convenient designs.
With the exception of witches and firebats, I have a soul shard for every other mob in the game, and I got all of them without using the anvil exploit, so I'd like to keep it that way. That said, I don't know if I will ever get around to getting my witch shard all the way to tier 5, but oh well. I probably won't be in this world for much longer anyway.