A new exhaust system will need to be developed to be able to use nitromethane effectively. Nitromethane has different gas densities and exhaust temperatures.
Nitromethane will require a different timing curve and combustion chamber shape. Nitromethane will require different port timings to work optimally.
In summary: Decide what percentage of nitromethane you are going to use and then develop the whole engine to run that mixture.