IMO, i would leave the 4L60E in it. 3.06 1st gear ratio plus a .70 overdrive for cruising with lower gears. Then you would'nt have to mess with the VSS (vehicle speed sensor) in the tail housing. Yes you can swap out the OE sensoers from the 4.3L. The biggest challange will be the fuel tank stuff. 45-50psi of fuel pressure down to 6-8psi., Cooling fans will have to have a seperate temp switch now. OE is ran off the computer. There is much more to it.