I imagine they used 12V body ground back in the 60's, but thats the only thing you'll want to double check
The thing that might get you is ignition / alternator whine ... there wasn't much push for noise supression back then as there is now (computers and what not)
Sometimes filter caps help with this, sometimes they dont, and you need a noise isolator ... I dont know how they work, but they work well ... my old 85 wagon was real noisy, and the noise isolater cleaned it right up, it installed between the line out on the radio and line in on the amp.
your head unit (radio / cd player) should have a +BAT lead, a GND or BDY lead and a +IGN ignition lead ... bat and gnd go to their respective places, and ign goes to anything that gets turned on while the car is 'on'
be careful not to confuse +IGN and the +ACC lead which is for accessories to be turned on by the head unit ... if your amp has a remote turn on connection, you could connect +ACC from the head unit to the amp's remote turn on... if not, use the ACC to toggle a relay controlling the amp
I seem to remember my Pioneer used red for +BAT, yellow for +IGN, blue for +ACC and black for body/ground.