250 fan belt help needed please

[Love 70 Chevys]

My truck has the same exact P/S pump and alternator stock positions as shown in the diagram. Could anyone help me with the cross section and length of the v-belt? A part number would be even better! A question I have is why are all of the pulleys double except for the P/S pump? Does the alternator pulley have enough wrap? I have seen pictures on-line where it looks line one long belt wraps the double pulleys twice. I am confused and obviously in need of help. Thanks in advance for any advice.