People go on dates with strangers all the time. While it can be fun getting to know someone, sometimes it can even be better dating someone you have known your whole life.
When you fall in love with your best guy friend you know weather or not you can trust him. First of all if you can't trust him then you would not be in a relationship with him right? You have a lot of memories together and see each other all the time.
The first time you go out with your best guy friend you could be total relaxed. You don't have to worry if he does or doesn't call you again like you do with a stranger, because you are already so close that you know you will speak. You don't have to put on an act and be someone your not because he knows the real you. You could say and do what you want because he knows you and if he is asking you out it's because he is interested in being more than friends.
There is nothing better than falling in love with your best guy friend. So if he tells you he wants to be more than friends and you like him go for it. If he is not your type then tell him how you feel without hurting his feelings. It may be scary at first but when it comes to love it is worth the risk.