Wiring is humbling. This is just my amateur boater's approach to this problem. There are many others.
My approach with our boat in that vintage was to rip out all the old AC and DC wiring, outlets, connectors and panels - everything. But this is my third boat, and over the years I've learned a bit about boat wiring. I found old solid untinned wire, corroded connections, deteriorating insulation, cobbled wiring that had been added to original circuits by different owners, and parts that did not meet today's standards. The engine was festooned with ground wires. I am starting over with properly coloured multistrand tinned marine wire, proper crimped waterproofed connections, and new outlets, connector boxes, bus bars and modern distribution panels.
I invested in a good set of current boat maintenance books (Calder and Wing, for example) and spent a lot of time cruising the Internet (especially the quality manufacturers' and distributors' websites).
I drew up a list of the devices I want to have on the boat, and their power requirement in amps. That gave me an idea how much battery I need. Personally, I separate house from engine batteries, so I worked out power budgets for each separately (cold cranking amps and running draws for the engine, and amp hours for the house). Then I figured out the maximum alternator the engine can handle (more amps = more strain on the engine). That resulted in cutting back my list of devices. I learned about battery technology, so I could make informed choices.
Next I figured out how I wanted to wire the battery charging system (shore power and engine alternator). I considered auxiliary power sources (solar, wind, generator).
Then I drew up schematics for the AC and DC systems to show wire runs, wire sizes, fuses and components, and checked them many times against various authorities, including ABYC, the Blue Sea online calculator and current standard textbooks, until I was comfortable that I understood the issues. I am still perplexed about grounding, but that's for another thread. Then I started buying parts.
In assembly, I try to follow expert advice about making marine connections, running wire and supporting wire runs on boats. I consult experts on critical steps, like getting an inspection before powering up the AC wiring for the first time.