Think I'd be shooting for something in the 400 ft. lbs. torque rating and lots of good low and mid-range power. Plenty of power for cruizin' with all the accessories running and with the o/d tranny you could gear it right and do some good highway running and keep the rpm in the 2200 to 2500 RPM range..... Much more power then that, and with street tires in a light backended pickup you'll just blow the tires off it anyway... No reason to go for 500 horse if you can't hook it up.... The lower ratio low gear in the trans will compensate a lot for the lower power numbers. Depending on rear tire size, should be able to get away with a rear gear in the 3.50 range.... Good acceleration and good cruisin' revs all in one package!!!! Sounds like a plan to me!