If these pickups are the same ones that are in the Sadowsky Modern basses, I once swapped out my Sadowsky Modern soapbars with Nordstrand Fatstacks.
The Fatstacks used the Bartolini P2 sized casings and they fit perfectly. You should be able to get the Big Singles in the same size casings.
The only difference that stood out to me was the drill pattern. On the Nordstrands there are 2 on top and 1 on bottom, so you'd need to drill two holes.
It might be worth asking if they can do the casings with the same hole pattern so you don't need to drill anything.