Mick - you're not totally off base, but remember there are two parts: Torque x Gear Ratio. To turn 2500 at 70 miles per hour would be pulling a substantially taller gear than the 2.5RS 5th. I don't question the engine's torque (mine makes peak RS torque at less than 2k... even at the very beginning)
Barrett - you need to be in here too... you figured out most of the how to of fitment of my car. I'm just working out the bugs.
And yes... 3.90 w/ the RA gearset is, I think, the right answer.