Regarding the harness colour, what about dyeing the blue one with a red dye?
I'd imagine to get both colours the same though might be hard but it's a cheaper option.
Oil temperature gauge is a very good idea for a track car, tbh it would be a good idea on any car imho. Fully synthetic oil can handle fairly high temperatures but I believe it's more about the oil viscocity as it thins out at higher temperatures that causes problems.
Cars coming along great.