why would you want that? You got a gigantic fuel tank on the Beluga as is. Put a 6D Fuel scoop on the darn thing and you are all good. More times than not you can with that gather enough fuel for another jump while aligning for said jump - or at least a large portion of fuel for said jump. All you'd do with more fuel tanks is to reduce your jump range.
Seems 2 me, that additional restricted slots should be for good suel scoop. Beluga - at least,class 6. Or 4 AFMU, if class 5. As I know, Beluga already have the largest fuel tank.
The way I do long distance passenger missions is with a Type-7 apart from a slot for one group of passengers I fill the rest with fuel tanks don't have to think about refuelling for at least 1200ly.
And I figured they allow cargo why not fuel tanks.
Anyway, just a suggestion. Quite happy to use my type-7 for this won't have to buy a larger ship.