You're going to burn things up with a FDR of 6.4 and a 3.5 motor.
Here's a suggested gearing guide from Novak for their Ballistic motors which may be a good starting point for you:
http://www.teamnovak.com/tech_info/b...r_gearing.html
I'm not familiar with LRP ESCs so don't know if the one you have has any profiles with timing but make sure you're in a profile with no extra timing from the ESC.
Invest in a temp gun, Pro Exotics PE2 or the one from HPI aren't bad and temp the motor frequently (every minute or so) when you're playing with gearing on that powerful a motor to make sure you're within the temp range recommended by the motor manufacturer.
3.5 is really a lot of power. When I stepped up to one of those from a 6.5 the difference was huge. I prefer a 6.5. The 3.5 is really a handful to manage and everything is under a lot more strain with that much power.