[LH Lead-Foot]

The grounds for any engine in a vehicle a important, yet with a high amperage alternator and electrical systems that use 5 volt DC is even demanding of a good ground path. Increasing the number of grounds and size are "Best Practices" following the negative to the engine, frame, fender, fame to cab (Under right door near fender) as well as frame to the bed at the back near the tail lights. Using the best copper strand wire and largest you feel comfortable with, will eliminate one more future issue. 3/8" Copper pipe can be drilled, smashed and shaped in you bench vise to install eyelets in these cable. I prefer to use shrink tubing, star washers and dielectric grease on these point of attachments.
If you are using the 4L60e (I have forgot), but they have a two wire VSS sensor that runs directly into the PCM. Provide an 18 gauge black wire mounted to the frame near the tail shaft housing, run it in the VSS harness to the PCM and exist the harness to ground again. From the factory, VSS sensors are permanent magnet generators (PMGs) produce an A/C voltage that is a small signal at slow speeds, but increase to 100 volts A/C at higher frequency during high speeds. To keep a clean signal, the wires are twisted 9 times per foot (Does not have to be perfect) but this helps eliminate mutual induction from low voltage / high current circuits & High voltage / low current spark plug circuits) As this circuit is assembled, include the ground wire along with the twisted pair, then if able, wrap with aluminum tape sold at home improvement store in the HVAC section. Get the cheap stuff as it helps as well with other harnesses that run close to headers / exhaust. Don't forget the front O2 sensors and the starter control wires that run up the back over the back of the engine. Use metal clamps to attach each harness so it is far away from heat. As you described, the drivers side harness runs close to headers, but tape it also, but decide if you can install a metal heat shield. like used on starters to reduce heat exposure. Doing this now, will help eliminate melted wires in the future.Even factory LS motor mounts have a sandwich heat shield that has fiberglass between two layer of metal. It adds to longevity of any type of mount used and can be painted with high temp paint to accent of disappear.

Depending on the many types of instrument cluster you choose to go with, the mechanical speedo cable is not needed. I found tab welded factory clip on the cab to hold it in place and routed near engine to make a large loop around the bottom of the park brake cable and steering shaft, then under the brake master into the firewall. You will now have a rubber plug with a hold in it that can be re-purposed.