There are many converters.... One end of the spectrum (Thankfully the large end) includes the progressive Dynamics 9180 with wizard I have.. (PDI's wizard controlled units are among the very best,, There is not one model I'd name "THE VERY BEST" but if I had to name just one, this line would be it.)
Back in 1978 the more common was a simply transformer and rectifier thing, no filtering, the output was rectified AC, not really DC and your radio will humm with that kind of power.. Drop in a new battery and it will learn the lyrics for you.
Or better yet, Upgrade to a Progressive dynamics 9235
If converter and power panel are one.. Upgrade only the electroincs and it's a 4600 model more than likely you will need.