re-build...?
Upgrade your air filter.. for a free-flow filter, preferably build a bonnet vent near the intake, or extend the intake to the grille (cold air reacts better than hot air). You'll need alot more oxygen (and fuel) in the engine when nitrous is added, and smooth transitions between the gas being on and off... better intake will improve this.
Make sure you meet the legal requirments (green sticker, dont exceed the speed limit with it...heh)
Try
Exhaust is also something to concider... all that combustion going on needs to go somewhere, if your exhaust is struggling, its just not gonna happen.. you need to clear the exhaust as quick as you put more fuel/air/no2 in. Not just a large backbox, but a full-system (talk to the guys at nos wizard.. they do a co2 kit too i think..?)
Safety, make sure you've got good belts, the bottle is secure to the boot install, seats (brackets) are in good condition and won't peel from the floor... just general high-speed stuff.
Also - look for a refilling station, in Weston-super-Mare (Nr Bristol) there's a place called "Stealth Engineering" that refils. You're looking at about £5 per lb. (an 11lbs tank = ~£55 per refil... so use it sparingly!). You'll start on 25bhp jets, and rolling road tests will let you progess to larger jets.. once NOS Wizard has approved the results.
Good luck, Im doing something similar on mine.
-- Ryan Gillett Peugeot 205 XS 1.4 TU3S Carb