Different locations around the country will dictate jet sizes. Generally speaking, depending on any other engine work done to the motor, cam, exhaust, etc., he probably should be running around a 45 or a 48 slow speed. Sometimes its a guessing game unless you have the good fortune of having a dyno available. Jets for the most part are pretty cheap which allows you to buy a few different sizes, replace and test drive and see which set gives you the best performance.