[Arran]

Nice, a 1960 Plymouth four door hardtop. Is it a Fury? As for the Predicta it could be dim for a number of reasons, the high voltage could be too low, due to power supply issues, the 1958 models used a voltage doubler power supply and if the electrolytics are shot the HV will be low as well, or bad paper caps could be dragging it down. It could also be that the picture tube is worn out, if it's an original tube on a 21'' set from 1958 this is possible as they had a short neck and a short electron gun, though most of those were changed within the first five years because of this. But yes, contact someone at the Alabama Historical Radio Society as Mike suggested.
Regards
Arran