i did a 4.3 to 5.3 swap it wasnt hard at all. motor bolted up to the transmission ( 4l60e) the exhuast ( stock ) bolted up the only problem i ran into was the mounts i used the passenger side mount from the 4.3 and used a 5.3 mount for the driver side had no problems..as for the fuel lines i cut the 4.3s and used flex hose to the 5.3 fuel lines..