I fabricated my own "Runtz" style voltage regulator/reducer. Do a search and it will surely come up with a photo or two. I also did as you and added a fuse block under the dash to handle the added accessories such as A/C, radio, PA system, and cigar lighter.
Pic of fuse box, relay, and power/ground bus. The (30 amp) relay is energized from key switch which sends battery voltage to power bus (I did not want all the added load going through the key switch). Most critical electrical loads have dedicated ground to ground bus which is grounded directly to cab, frame, and battery.
