It seems we didn’t have a safe place to the kill the wither (or at least everyone I asked said we didn’t). I made a place that’s just as safe as the one we had on the old map. It uses a specific bedrock formation I found in this video: https://youtu.be/cnTBfgt7Y0A?t=1m7s
It’s down the SW (pink?) highway.
Pictures and instructions: https://imgur.com/a/9PXws