electrical gremlins are back

Size of fuse is "up for speculation." The reason you are doing this is to PARALLEL the old black and the old red which now, instead of conducting charge current on the road to the battery, simply power "stuff" in the interior of the car. So the current requirements on the "new" wiring is now very much lowered, and of course the object is to protect the bulkhead connector

(On a side note you can get the new bulkhead connector terminals they are known as "Packard 56) terminals. Make sure you get the proper ones)

The one big thing you are trying to protect (with a main fuse) is the size of the wire and the terminals through which they go

Other factors is WHERE you mount the fuse(s)/ breaker(s). If you mount them in the engine bay, subject to high engine heat this DERATES the fuse/ breaker amperage