The 775 motor is about 1100 kv or 1100 rpm per volt. Your Titan 550 is probably 2-3 times as fast. Now, in order to get any kind of reasonable speed (assuming you are able to mount it in the first place) you'd need 14 volts and a big fat 5mm bore pinion, around 30+ teeth and a 78t spur or so.
Then you'll find you wheelie a bit too easily, at which point, as suggested, either stretch the rustler 1" or put some weight up front somehow to keep the weight balanced. Even if you don't wheelie, you won't turn very well with almost no weight up front, relative to the rear.
A VXL trade in or something brushless like that would be a better investment (as cool as it sounds putting a 775 in a Rusty).
