I don't know about who can and can't, the gov doesn't care about that. They care about safety & the environment (or so they tell us, but I'm digressing...). There are FMVSS sections that deal with the construction of gasoline and diesel fuel tanks and or if they can be inter-connected. The TransferFlow web page used to make a big deal about what the tests are and how well their tanks do in the testing, particularly in the drop test. Might have look if interested in that sort of thing.
Then the EPA gets their say in things. If dealing with an emissions controlled vehicle the tank will need to have all of the appropriate connections available. If the vehicle in question is a 1996 or newer model then it will need to have all of that or the computer will throw a fault code. Finally, and my info is sketchy on this so investigate it first, but the charcoal canister may have a tank size range where it works and if that range is exceeded a new canister for a bigger tank may be needed. I've not seen much said about this, but I have run across it in a couple places. No idea what the physical and legal constraints are.
All of that aside, some considerable though needs to be given to baffling the tank and creating an internal sump for the fuel pick-up or partial fuel level combined with vehicle angularity or high enough lateral G's can result in fuel starvation. If this is an EFI system that will be near instant death to the fuel pump as they need fuel in them for both lubrication and cooling.
