4.3 to. A 5.3 swap

Hey everybody was looking into doing a full drivetrain
swap for a 2012 GMC Sierra WT v6 unfortunately and
putting the 5.3 with the  matching transmission..... what
else would I need to worry about in regards to complete
the swap successfully. I k ow I need the motor
transmission which you can get the set already together
drop in ready but what else do I need.... would I require a
new harness? Computer? Anything helps other than
saying go buy a v8 truck which is not an option looking to
do the swap as a project with my wife