When hanging larger cabinets I’ve had to deal with the walls not being flat or plumb. Battens along the bottoms of the cupboards appeared as a safer and simpler alternative. Please see photo. The inset pane just shows a way to support a wide cabinet while mounting it. A rail along the top back allow for screwing the cupboard to the wall, without those screws having to take the weight of it. That also allowed for using a thinner back, flush with cupboard back – somewhat increasing the interior depth.
Would starting with the pins of the dovetail joints be an idea to be considered? Marking out the pins from the tail parts could perhaps call for holding the tail parts 48″ up in the air. The second photo shows the marking out the tails on a 60″ long piece of a sideboard.