Zex is not the only one. NX, N.O.S., any WET nitrous kit will work. just make sure you get the correct jets.
everyone just sells the Zex kits becuase they are a good kit for a beginner, and everything is all in one little box.
a 75 wont hurt anything. you could spray more than that up to 125....with a healthy motor for a while. once you get above 100...you might want to start retarding the timing....the more you spray...the more timing you want to take out.
I have a 150..I take out 4degrees......
the nitrous will cool the incoming air into the blower as well..so this will help out A LOT!!
I can see .5sec drop....