Dual to single pulley
 

Now that my air con has been gone for years and I'm staring right at my double pulley would there be any performance gains in going to a single pulley?
I have one from an earlier car and it is a bit taller so hopefully I can change the shims to make the belt longer?
Does this mod worth spending time on or should I do more important things like finishing my engine?

Finish the engine, and you would need a new belt , I doubt the shims will help if the pulley dia is different.
Alan

If you fit the smaller 116mm pulley you will effectively slow down the fan and the alternator by around 12%.

This will reduce parasitic losses due to windage but may reduce cooling a little in hot weather.

If you use this crank pulley with a later fan you will need a belt with a length of about 695mm.

The largest pulley was used on the 77 to 79 with the like fan of the Carrera.
Install that pulley with a belt 9.5x725 you'll be good to go.
Bruce

The Single pulley I have is slightly bigger so would it not increase my fan speed?

Are there any advantages to the lighter pulley?

I always associate 'early' pulleys with the 116mm diameter of the early engines and these have a smaller effective diameter.

If you have a larger pulley then it will be faster:)