67Dart273;1720439 Bill I don't understand exactly what you did saidSorry, "back-to-back" wasn't the best description. The diodes are in parallel, facing opposite directions, so one is in the forward direction for flow either from or to the battery. The ones I found on ebay (photo) nestled together nicely to bench solder together. I then filed the holes clear for a #10 screw.
You might use the stud-type diodes that have wire cables. I didn't see a good way to mount those in my setup. You would need to electrically isolate all exposed metal since at battery voltage so you wouldn't want a wrench brushing the metal. My first thought was to put diodes across the firewall leads but harder to dissipate the heat there, plus would have to run 8 awg wire to both sides.
A final comment on conductor sizing. Many people think 12 awg = 20 A max, from their knowledge of home wiring. That rating is for wires buried in fiberglass insulation, and the concerns of burning down a house. In a car, the "appliance rating" is more relevant. I recall that is more like 40 A for 12 awg.