the difference between 1/2 and 5/8 is barely anything. If you are set on using 1/2 tubing, just dip the tip in boiling hot water to soften the tubing, then slide it right over. You will have a nice tight fit.
I do it all the time to put 3/8 tubing on 1/2 fitting. You can techincally go clampless this route. Just a thought