The RPM signal should not be a lot of current and the Arduino output itself (maybe with a resistor too) will likely suffice.
As for the 0-12V "VSS" - Is it PWM, or some varying frequency or what? You're going to fool your panel's speedo that it's getting 0-12V or some PWM-related signal? That's probably not a high-current deal there either.
Anyway, leave that IRF510 at Radio Shack.