If you fuel from a syringe, you can short-tank the bigger tank AND take advantage of the forward shift in CG towards more stable. Then swing back to the Babe Bee for a truly shorter tank/duration.
The real trick would be to get the CG right for the short tank, and flip the plane to keep the same CG with a bigger tank. That could be as simple as a spacer with the Babe Bee, and no spacer for the GB/BW with longer tank.
In the end, I'd recommend filling from a syringe where you can observe just how much was put in the tank short or full regardless of which engine/tank.
Enjoy the build...enjoy the flights!