There are two ways to get the rear drums off. The screws are the preferred route, since you don't have to mess with the grease seal.
The other way is to pull the axle(s), which will allow you to remove the drum and hub together and work on the screws on the workbench. Start by removing the 8 bolts on the hub, which bolt the floating axle to the hub. The axle will then pull straight out, but will probably needed some massaging with a hammer to break the gasket loose. Inside are two round nuts with a retainer between them. Bend back the ears to clear the nut. There are special sockets for these nuts, but a narrow screwdriver and hammer can be used to turn them. After both nuts, retainer and washer are out you are ready to remove the hub and drum. It's heavy, and pulls straight off.
Also, if it isn't removed properly, the grease seal will be ruined, but it will need replaced on a truck this old. You can get a new seal at your local auto parts store, it was used into the late 70's.
When you go to put things back together, it will be easiest to have the drum separate from the hub. Slide it on carefully, or you will cut the lip of your new seal. Then put the nuts back on, followed by the drum.
It is recommended to use a thin gasket between the drum and hub. I usually use a coat of anti-sieze instead, it's there to keep things from rusting together. Clean up the threads for the flathead screws, and assemble with red threadlocker. It will keep the threads from rusting, will stand up to the brake heat, and will allow you to hit the screw head with a torch to break the seal and loosen.
It is also highly recommended to get the reprint of the factory repair manual, which describes how to do this kind of work. Cost is about $25, available at most any of the old truck parts retailers.
