[flatheadmurre]

The cheap converters are usually lacking 2 things enough heatsinking and an isolation transformer.
The lack of the transformer means that they will be 12v Pos ground...so even if you get the cables right on a modern radio as soon as the metal case hits the dash sparks start flying and it shorts out...since - is now hitting the grounded + dash...
I use them to both for Dropping and boosting just be aware of what you are buying...the ones with isolation between input-output has a higher parts count and cost more...