It's hard to believe that no-one other than @Twin Jimmys has actually encountered this & done something about it - there are dozens of outfits offering this booster/dual-master kit and just say 'may need the crossmember' but it's clearly not the whole story
As Hot Rod Lincoln mentioned above, the more common solution is to move the master cylinder back, extend the rod, and leave the crossmember alone.
On Edit: Your solution with the universal crossmember should work just fine. I would be inclined to put the 1" spacers on the sides like you mentioned to avoid reducing your ground clearance.
