I would not want the motor out of my WRX in my truck.
Not enough off-idle torque, then it all comes on in a big rush right around 3000 RPM.
Freeway mileage in the WRX is about 25-27 if I put the cruise control right at 70. Any faster than that and it drops off quick. A truck is a lot less aerodynamic and big wheels/tires have a lot more rotational inertia so the mileage would probably be considerably worse.
I'd much rather have a large, torquey V6 or V8 with plenty of low RPM oomph. A GM 3800 V6 seems like it would be a good choice. Anyone ever used one of those?