Here's a rework of the 1960 main power.
I don't know the 1960 wire colors and sizes so I guessed or made them black. I think its helpful to understand how it was originally set up.
View attachment 1715890405
Then we can figure out how the changes fit in.
The maxi -fuse is in the A1 line. That's a reasonable safety change. 40 amps might be more appropriate unless there are some very poor connections or wires downstream.