Made up the battery (-) ground wire using the stranded aluminum wire I got at the scrapyard the other week. The piece ended up 40% lighter than the copper wire. Not much actual weight removed though, as the original ground wire wasn't that heavy to begin with, but I needed a practice wire to work on before I tackle the main + line from the battery. I made my own terminals from aluminum tubing, as I couldn't find any aluminum terminals at a local electrical warehouse. I sanded and brushed all the electrical surfaces to remove any oxidation, and then crimped and welded (via a couple drilled holes in the terminals) the assembly to hopefully get a decent electrical connection. I then just taped over most of the interface area to slow down future air leakage and oxidation.
I have one end of the larger diameter positive battery-to-starter wire finished, but it looks like it will take some time to extract the original cable out of the tunnel area, so this portion is on hold until I finish a couple track days I'm trying to squeeze in this month.
In the mean time, I'll make another short aluminum replacement ground wire between the starter housing and the frame. This will only be another few ounces off the front of the car, but it will at least remove the annoyance/distraction factor that hits me occasionally when I'm on the track and I keep thinking about unnecessarily heavy items on the car. Last week I did take a few pounds off the front of the car with modifications to the steering rack mount, the alternator bracket, the water pump cover, and a frame crossmember.
I did take an additional 12 temporary pounds off the car a couple weeks ago when I pulled the passenger seat off, and bolted on an additional fire extinguisher bottle/bracket for this month's track days.