I'm going with an electric fan to replace bad fan clutch. What changes need to be made to belts and pulleys? I think I read about having to double pulley on alt because of slippage? I have an 1980 with AC.

 

No changes needed for the belts. You will need shorter bolts that secure the water pump pulley because the fan clutch extension is not needed. If you don't use shorter one, they can bind the water pump.

 

Or leave the extension on.

For my stock 12A, I left the air pump on, mainly because rules don't let me remove it.

In practice, once you remove the clutch fan, you don't need a double groove alternator pulley or anything wild like that. The main load on the water pump pulley is the fan. You need the double belt if you have the fan and you remove the air pump.

Caveat to that is that if you rev over 6000 long and often, alternator belt life is short because the water pump cavitates at speeds that high and this shock loading eats belts.
On my much higher revving 13B, I tried what is colloquially called a yoohoo belt (only goes around the crank and water pump, very short). Belt life actually got worse because the belts fought each other. If you get a double row alternator belt you really need a PAIR of belts, made to exact tolerances, rather than two belts of the same part number. I'm not sure that they even sell them anymore.

On THAT engine, I just got an underdrive crank pulley and belt life became close to immortal.