You don't see it listed because there isn't one. The 69 Camaro big block engines all came with the smog pump and for the L78, the belt went around the crank pulley, the alternator pulley, the water pump pulley and the smog pump pulley. The AIM shows the routing on UPC L78, sheet A6 and the belt was P/N 3921934.
(By the way, the 63 amp alternator wasn't available on the L78.)THIS
is the belt you need, but again, you have to have the smog pump installed. (And save your money. From a very reliable source, these belts aren't very good.)