RE: 1/6th A-6 Intruder Scratch Build
Be VERY, VERY careful using Variacs. Some are VERY dangerous. Some, not all, reduce the voltage between the output terminals but the voltage at these terminals is the SUPPLY voltage to earth & you are at earth potential.
What I use is a 240 volt (Oz mains voltage) to 32 volt transformer of a suitable current rating, about 5 amps for Oz, perhaps a bit more in 110 volt land. I drive the input with a normal domestic light dimmer (the type suitable for transformer powered downlights.)
Do you have any truck driving friends? If they have a 24 volt battery charger that will also work fine. - John.