I also agree with Petetotally...short of a fire, there is little reason to replace the harness...but after 40 years of undoubted neglect, returning the connections to as new quality will go a long way to making the car reliable and preventing problems...I suggest you lay in a stock of anti-corrosive zinc paste and start there...see: http://sw-em.com/anti_corrosive_paste.htm
...after that, a few improvements would the next step...if you are running original lighting loads, there is also little reason to complicate the wiring system with relays, but if upgrading them, you must consider the WHOLE current path...see: http://www.sw-em.com/lighting.htm

Cheers from Lignano!