I agree about using pre-tinned wire for any circuit rewiring. Look for it in any proper marine chandlery. It costs a bit more but is worth the corrosion resistance. I also use di-electric grease on any ends of terminals or electrical fittings, especially on bulb sockets and wiring ends exposed to the exterior of the boat, ie, running lights. As far as the wiring on a 1972 era C 27, remember that the wiring was rather cheap to begin with to save on costs, and it is now 40 years old! I'm only 2 years older than your boat, & were all getting older. I like to think we are better with experience, but not when it comes to marine corrosion. Fiberglass is so durable that we sometimes forget the age on all of the other essential components on our Catalinas. Most sailors have rewired and re-rigged just about everything that was original and thereby questionable on our boats of any 30+ year vintage. Worn out & unreliable wiring circuits are a warning sign. Don't let a short turn into a fire source. I've seen the results of boat fires in Deltaville, VA. Burned 3 (almost new) 40 ft + yachts to the waterline. Yikes.