The mail confused us about the positioning of the three beam chambers. The downstream one is mounted as close to the TPC as possible along with the beam trigger counter (and must be removed each time the trigger is changed. This unit is 0.3m long). The mail referred to the two upstream chambers being separated by about 1m which puts the second one closer to the third than the first. In our design discussions, we assumed that the second chamber could go quite close to the first and that the entire assembly of the two chambers is 0.57m thick (10 cm for each chamber and 37cm separation).
We then worked with the remaining 1.4 m for the target changing gear (thereby allowing that the first two beam chambers would not be dismounted at each target change) for the new design. This is just enough space for the long targets. See Chris's diagram of all of this.