Go with the T56. You have most of the stuff already there since you have a 5 speed. The T56 is stronger than the 4L60E and it has a double overdrive so you will get a lot better gas mileage with that 383. There is someone on here who has done the swap and has a little writeup about it. I know you will need an LT1 flywheel and starter. Other than that im not sure.