My AC system on a 261 ci motor (very similar set up as your GMC) went like this; use a single belt to run water pump and also compressor which is now mounted where generator was. The compressor has a double pulley, this is where your second small belt comes in, from second groove on compressor use short belt to alternator which is mounted directly above ac compressor. Even with ac off, this set up will drive alt/water pump all the time. We made our own brackets, one to mount compressor and other to locate the alternator which is above the compressor and to the side of your thermostat housing. There are several vendors now that have the brackets for this motor, check Alan Grove and possibly Chevs of 40s etc.
Best of luck
Take a look at this set up, pretty much like mine.
http://www.alangrovecomponents.com/images/New_Images/Large/301LTrial.jpg
