The battery pack is there primarily for acceleration, and to recapture braking energy. You get about 2 minutes of a trickle ( 10 HP ) help, or about 20 seconds of a 40 HP boost. ( It's there for passing on the highway. )

You get electric motor help all the time, but it's not from the battery.
The gas engine has an ideal range, and in this range, it will actually turn the generator, and the generator will send power to the electric motor, and the electric motor will help turn the wheels. This sound weird, but this is exactly how every train locomotive works. A diesel locomotive's engine does not ever pull the train. In that case, electric motors do all the work, since they are much more efficient, especially at low speeds and high torque demands.
-John
P.S. 1400 pounds will be just fine.

P.P.S. Don't get hung up on horsepower though. Torque is more important in your case. A John Deere tractor may be 40 HP, but will pull out a tree.
A Porsche may be 400 HP but won't pull a toy wagon! The torque always quoted is at high speed. The Ford Escape Hybrid has about 500 ft/lbs of torque taking off from a stoplight. How does that sound?