I have a 72 cutlass at night with the lights are on and the ac the amps go down to about 10 volts at idle step on the gas and it goes back up to around 12.5 volts.do you think i need an upgrade of an alternator.and if so what size.i have a 455 motor but was upgraded from 350 motor that had ac.In drive the idle sits at 800 rpm

 

If only the lights are on and this is occuring, it sounds like more of a regulator issue, or maybe the alternator is failing, or there are connection problems.
An 'upgrade' is not really needed if the car is stock.
If you have A/C, then you should have the 63A alt (with separate regulator). However, even a 35A will run the car and lights with no issue if the A/C is off.

 

I had the same problem with two different 71 vehicles, a GMC and the Cutlass. I tried different voltage regulators, both mechanical and electronic type. I cleaned all grounds and still the lights would dim. A later model alternator with an internal regulator set up for 1 wire cured the problem.

 

I could even be that the wiring just can't hack it any more. When I replaced to 40 year old wiring in my 442 last year the brightness of my headlights more than doubled.