You shouldn't try to get back to your flat bar geometry because road bikes are designed to be more stretched out (and the brake hoods allow you to do that comfortably.) How tall are you and what's your cycling inseam? If your seat height is set properly it looks like you are pretty tall (~ 5"10") for that 18" frame so you may not want to start at 70mm. Also flip the stem already, that much drop could cause discomfort.