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The first szenario is describing exactly were I´m going to... and yes, you are right, increasing the length of the y axis is required, because of the rear belts crossing the area in front of the rear extrusions. It might be helpfull for the x axis length as well. Last to increase the number of tools.
Two´more things to check:

  1. the tool changers nozzle hight to the highest level the bed is able to move, or compared to BLV´s original design
  2. changing the x carriage for BLV´s belts alignment