What's the best modification you can do for top end speed aside from nitro or headers? Thanks

 

Headers are actually for improved torque, and despite F&F, nitrous really shouldn't be shot for extended periods of time - like when you're trying to achieve top speed. Your best bet would probably be cam(s) with longer intake and exhaust durations along with titanium valve springs to prevent float. Your motor needs to be stronger in the high RPM range.