I would start with a 310 main and see how it runs. Be sure to do several plug checks to be sure he's not running lean. As far as the clip position, I'd leave it stock at first and adjust it once you have the main jet perfect...you may not need to adjust the clip position at all. Always make one jetting change at a time, so you know exactly what is doing what. i.e. change the jets, run it. If it feels good on top and the plug looks good but has a slight hesitation or sputter in the mid range, change the needle clip position and then run it. Just take it one step at a time and you'll be fine