Hello,
This was my solution to the same problem.
The cleanest sounding auto sound system I ever heard was the one I installed in my old 1954 Chevy panel,the problem was it had a 6 volt system and I wanted to install a 12 volt 100 watt power amp and a AM/FM cassette. I wanted to keep the original 6 volt system, so I fabricated a bracket on the side of the engine, mounted a 12 volt alternator with a built-in regulator lined-up the pulleys,ran one belt around the 6 volt generator, water pump, crankshaft and 12 volt alternator. Fabricated a 12 volt battery mount on the fender skirt, grounded the 12 volt battery to the frame, ran a cable from the alternator to the positive side of the 12 volt battery, ran all sound system power directly to the 12 volt battery positive and negative posts. The sound system had ZERO 12 volt alternator and 6 volt generator noise and interference,even on AM stations.
It was a hybrid 6/12 volt system sharing the same ground. The thing that was nice, with engine off, I could run the sound system until the 12 volt battery was dead, then just hop in the truck and start it up with the 6 volt battery and a lot easier than a 6 to 12 volt conversion.
A 100 watt power amplifier at full volume draws a lot of amperage, so I needed a 12 volt battery source so the the amplifier was not starving for power. You could use a 6 to 12 volt convertor if just using a radio, but for the same cost of a convertor and a Memory Maintenance Module you could buy a used junkyard alternator and battery. This is the correct way to do this.
Most 6 to 12 volt converters have low amperage output and would not supply the correct power need for a external power amplifier.
Also Modern aftermarket automobile stereo in-dash radios CD/cassette Head Units require special wiring.
Otherwise your battery will drain down dead.
Read Here For the Special Wiring Info To prevent dead battery and to maintain radio presets and time.
The 12 volt alternator and battery add on system can easily be removed to bring the truck back to its original electrical configuration.
If you have any question let me know.
Thanks
Tip: you can solid mount the alternator and use the 6 volt generator belt tension adjustment to tighten-up fan belt.