Interesting thing is, my 79 shop manual shows a final drive of 15/35, which is exactly the same as a 18/42
But if you crunch some numbers, you will find, that running a 34T rear sprocket with a 18T front sprocket will make your CBX act like you are taking off in 2nd gear all the time:
First gear ratio is 2.438. With a 18/34 (= 1.89) final reduction, you end up with a total of 4.61 ratio (Stock would be 2.438 x 2.333 = 5.69)
The stock bike in second gear is 1.750 x 2.333 = 4.08
I am not sure, if I would want my clutch to do this all the time.