I would highly recommend a 4L60e or 4l65e to match the engine. The radiator is fine but you'll need an electric fan if you go LS1. You can use the manual factory fan with your old stock shroud if you must, if you have a truck engine with the screw on fan. I ran the 28" radiator with LS1 factory fans without any issues. I got a new 34" rad for the NBS fans but really didn't need either.