If the 12v
is in good condition, reuse and leave in place as much as you can. Otherwise you can open up a real can of worms. Things tap and branch off of each other everywhere and tracing it all in the amount of wiring that will be there will be daunting. I reused the vast majority of mine, when I did want something extra I ran some additional 12v
circuits. I cleaned up what was there and made sure each connection was in good shape or fixed it if it wasn't. Good amount of soldering and heat shrinking. New fuse panel and converter are good starting points for planning.
For the 120v, go to town. There wiring is more straight forward and there is much less of it. You'll probably have 3 branch circuits that are there originally, but in 1966
there wasn't a lot of demand for outlets for personal electronics, TV's etc. Think about where you'd like outlets for added functionality and run as appropriate.
If you think you may want it now or in the future, plan to run some dedicated wire for solar panels.
Think and plan a lot ahead of time because once you put the walls back in things get much harder to change.