There are a lot of good vendors, but not all carry the same variety of parts. I have requested catalogs from most of the classic parts vendors and sometimes have to go to 2 or 3 of them to get everything I need if the parts are unique to the squarebody. Also, since its a GM vehicle, I find the catalogs for Chevy cars (Camaro, Chevelle, etc.) to be useful if you need GM hard parts for the engine, transmission, etc.
LMC Truck probably has the largest number of 73-87 parts in their catalog, but its hard to tell what you are getting until it arrives. Their online catalog has very little information on the parts source, and they do not provide the detailed information you might need to make sure a part fits your specific truck. I've found Rockauto to be a good source for most standard parts, and their prices are very reasonable. In most cases they also have useful information about the exact parts vendor and specs of the parts you are buying.
Bruce