[LS short box]

For the LS wire harness you can pull most of the wires to the back of the engine. They will fit under the plastic coil covers.
The chassis harness to the front I ran down left frame rail.
I used used a Painless universal chassis harness. I filled the the stock fuse block hole. Then moved the Painless fuse block back a bit and then made a small hole you can't see to run the front chassis wires forward along the frame rail. The Painless wires are plenty long enough.
Here's a pic of my current 80 Camaro LS3 swap.