I've done a few square body ls swaps.
Unless you have tons of extra time, buy a painless harness.
With purchase they will unlock and flash your stock ecm.
Great customer service and there is no way to make a harness simpler.
I have a 76 gmc sierra. 6/8 drop. C notched. 9 inch. Ls swapped. 4l80 swapped. Leather interior. I want to do something different but not change it's driver ability....