It will be interesting to see how they propose to integrate a boom system onto the 390 - if it's centreline, (can't see any alternative tbh...) I would imagine that the ramp and door would need significant structural modifications, and that it would become a dedicated tanker as the ramp and door would effectively be redundant. Assuming internal cargo hold tanks as well to facilitate greater fuel uplift/offload, any servicing requiring removal of the tanks would then require the removal of the boom system.