Don't be misled by horsepower ratings. A 120hp motor is gigantic - like 900lbs gigantic. Your truck doesn't need that kind of power. A large vehicle-duty motor will deliver 120hp for the brief period that you need that much power, like during acceleration. But before it can become completely heat-soaked and destroy itself from overheating, you'll have achieved your desired speed and backed off the throttle.
I think the power rating you're looking for is more along the lines of 50-60hp.
http://www.go-ev.com/motors-warp.html is the link to NetGain Tech, they've been dealing EV motors for a long time and provided the two 8" motors that drive White Zombie, a record-breaking EV dragster. Check it out.