No it is not legal. But you can stretch the rules. It is illegal to have a nitrous bottle connected in your car, but it is not iilegal to carry a nos bottle in your car. It is legal to have nitrous connected in your car, as long as there is not supply bottle connected to the system.
SO..........................
Put in the nitrous system, put the supply bottle in the boot, get it painted red with a black stripe ( to look like a fire extinguisher ), and have the supply line tucked out of vision, when you want it on, plumb the supply line in, turn on the tap, and hold on tight.
.
Something else you can consider, althoug more expensive, edlebrock do a plate system for the windsor, that bolts between your block/heads and your intake manifold, and all the plumbing comes out the back of it, impossible to see without removing the engine. You could then use 3 smaller bottles, located under your seat ( not like in fast and furious) on the floor, with a filler valve. This way the whole system can not be seen, and you can fill the bottles without removing them from the car.
I think those plate systems are about $1800, but can save you lots of harrasement from the authorities.