UPDATE - 7/11/18 I updated the drill guide to V2, this moves the mounting holes for the belt retainers out 3mm to better align the belt path. Thanks to thisiskeithb for bringing this to my attention ( I didn't use the drill guide for my mod originally ).
The stock Y axis on the Ender 3 has some issues, the softness of the aluminum plate makes wheel alignment pretty difficult. The wheel spacers will tend to dig in causing the wheels to bow. Even when vwheels work well I don't like them for Y beds. This fixes that.
Re-used existing bed plate, my idea is to have a 3d printable drill guide that registers with the vwheel mounting holes so anyone can do this easily. I used a 300mm MGN12H rail for this. The Y axis switch was relocated with a spacer. The stock Y belt is re-used and attaches in the same insert-into-slot fashion using the printed belt holders.