VERY important...do you plan on keeping the stock radio. IF you have a factory FM radio, do NOT do the move.
I'm sorry but I don't have the specs you need on mounting. A factory assembly manual would sure help at a time like this as they show dimensions/measurements on stuff like that.
If you keep your stock radio, you'll have to "trim" the antenna so the reception is optimum. I can show you how to do this later.
Yes, in '66 you could have gotten a rear power antenna. It was the only year for that option.
'66 AM radio owners could have gotten:
The standard front telescopic antenna,
Rear telescopic antenna, Rear telescopic power antenna
'66 FM radio owners ONLY got one antenna, a front fixed-mast antenna.
Your antenna would be mounted on the passenger side though regardless of front or rear.
Joe (building this site's radio pages)