My preference is to wire in a completely new modern fuse panel in a convenient spot and feed it direct from the battery with fusible link/fuse/breaker protection between batt and new fuse box.

Most of the modern stuff we add draws a fair amount of current and I wouldn't trust it to a 40 year old harness or fusebox...The bonus is that if you wire all your new goodies to a seperate box with it's own main feed, troubleshooting is pretty easy.

Don't forget to upgrade to a bigger alternator, think about bypassing the factory amp gauge, and make sure to run a larger main charge wire direct from alternator to starter relay or battery.

I used this one when I wired in extras to my 68 charger.