peak torque on the 351D E and M V-6's is 1600 rpm. you'll have to have the accelorator on the floor to keep it moving at that low of an RPM.
i would be constantly shifting i think or getting pissy with it dogging out all the time.
But it is a low rpm low compression truck engine so maybe it rather be trunning 2,000rpm at wide open throttle than 3,000rpm at half throttle.
what about a 5.9L diesel with a 6spd ?