a/c charged and cooling

[kglowacky]

well today was a big test for me and my truck. After driven it for 2 summers in Houston without a/c. I decided to see if an old factory a/c system with only a new type compressor and a orifice change would cool. Well it did with a couple challenges still left
Info. 78 c-10 Silverado, new r-34 compressor and condenser. 2002 Camero ls 1 and 460 e.
Here are what I am trouble shooting.

idle is good and when I turn on a/c it adjust perfectly
cold air
had to bypass a new binary switch. Going to go back to the low pressure oem typed switch. hopefully it will work.

Then my mechanic put the truck on the computer, and all was good except it threw a alt. volt code. At idle no a/c 13 volts. Turn on a/c drops below 13 not sure but my guess 11.5 sometimes it will come back up at idle sometimes it doesn't this is in drive sitting still. Go down the road it goes back to 13. He thinks my alt needs rebuilt. I thought I read somewhere on here 12,8 is normal.

last my engine temp running 181. He thinks too low. I have it hooked up when turn on a/c cooling fans come and run all the time. I also have a big, oversized radiator. I am old school and always believe the cooler the better. He says maybe running to cool and may affect my gas milage as it is around 10mpg,

last on MPG I have a 3.70 rear end ratio gear thinking of going to a 3.07. Any thoughts how much this will improve my milage?

Thoughts???