That's how mine goes also, 64 1/2.

 

Two Groves on the crank and water pump. Three on the crank with AC

 

Here is how mine worked out with aftermarket air.

 

Here is a general guide for early Mustang v-belt pulley arrangements. On all of them the fan (water pump) pulley is just a single grove.

 

What you have noted there they show for the 302 or the 351. No mention of the 260 or 289 which were on the 65-66 Mustangs.

 

In 65 with A/C the Alternator got a 2nd groove as well. The Engine assembly manual shows the configurations. I can check it tonight

 

I know the OP's question was about 65s, but did they change that in 66? My 66 only has a single groove for the alternator, however my A/C is aftermarket.

 

64 65 66 Single groove Alt pulley.
PS ran straight to crank, back to pump. NOT over water pump.
In 67, Yes, there was indeed a 2 groove WP pulley for the PS. Alternator ran on first groove, PS on second.

 

Picture from a long time ago on my car. 3 years. Early Eaton pump but same idea.

 

As usual Chock is correct.

From the 66 Engine Assembly Manual

289 Engine with Power steering

Alt, Water Pump, and P/S are all 1 sheave (Groove)
Crank is 2 sheave

1st Belt goes between Alt, Crank inside Sheave, and Water pump

2nd belt goes between P/S pump and outr crank Sheave

The 2 Sheave Alt pulley is used with A/C to drive a higher output alternator