I put in a 350 with 4L60E in my 58 Chevy half ton from a 94 Buick Roadmaster. Used the ECM from the car along with some of the wiring.
Still has the 3:90 rear gears. It is fuel injected and all computer controlled. At 70 mph I get right at 20mpg.
But, from a dead stop I have to watch how much gas peddle I give it when I take off. Wet roads can really get fun. But on dry pavement once you are rolling a little bit, you gun it it goes.
It will turn the truck around in a heartbeat if you get into the throttle because the rear is so light. (Mine is a short narrow bed)
