But-but-but.... how do you power your radar detector on 6V????
(Only half-joking: some of those 1950s Studebakers were awful fast at the top end.)
I've mucked with DC-DC converters enuff to know that $100 is a fair price. It "should" be $50, but demand is so very low that $100 is fair, and less total strain than perfecting a DIY.
Sniff around the DC/DC module catalogs. You might find a 100W brick for less than $100. (Yes, the demand of a modern dash-box can approach 100W.) And it might not throw too much hash. But most are either 48V in or 3V out. 6V-12V may be rare, especially at surplus price.
Yes, isolation is probably needed to run the wrong ground. It is possible to float everything, but neg-ground is so standard that I bet most radios have something neggy bonding to the chassis.
Vibrator and 2*6VCT transformer would work, and is "period", except most vibrators were only good for a pair of 6V6, say 30W max.
Too-obvious: You could just charge a 12V batt at home and run the radio on that for several hours.
Flipping the polarity on a '53 Studie is fairly trivial. Nothing cares. (Motors and gauges still go the right way.) But still 6V. You could stack a second home-charged 6V batt for a +12V line.
Change-over from 6V to 12V is so obvious, so convenient, and generally so simple, that I assume you have reason to cling to obsolete voltage.
It was sometimes done to run a second dynamo for added loads. The hassle is that the classic crank - generator - fan/pump triangle belt plan will not accomodate another load, so you must hack a second crank pulley as well as your alternator mounts.
It was also done to use a DynaMotor. 6V motor and a generator. Most often the gen made high voltage for plates, but all outputs were available. Few can be found today. A 6V generator can be a motor, and a 12V generator coupled to it. I guess two dynamos cost more than $150. The whine is distinctive.
I suppose you could drop a 12V generator in there (even a 12V regulator on the 6V generator and accept no-charge below 1,500RPM) and rig a monster voltage-dropper. A depleted battery can want 20A or more, 6V drop at 20A is 120 Watts, and this is a very not-1953 hunk of aluminum fins. (Hummmm.... you could hide a heat exchanger in the lower radiator hose: battery-refill tends to happen while the engine is still cool, and modern sillycon will stand 212F.)
It occurs to me that a standard PC power supply main transformer has 5V and 12V windings, both typically CT. The CTs are common-ed on the PCB but typically not in the transformer. You could pull it out and attach push-pull transistors to the 5V winding. Keep the whole 12V rectifier and caps. Nominal output is 12/5 times the Studie's voltage, could be 17V at cruise, so you want a duty-cycle control to sag it toward 14V. Regulated seems nice but a fixed ~~84% duty cycle would let the "12V" track about twice the Studie's voltage, which is acceptable. I bet that perfecting this (including filtering the 23rd harmonic off the AM band) will be $150 of brain-pain. Keep your fingers off the original 300V winding!
As long as you are voltage-converting, and hanging on a DIY audio forum, why not kick-up some 300VDC and run a bank of 6V6? You GOT the heater power right there. A tuner and CD drawer will run a long time on a 12V tractor battery.