Here's a link to the
1948-1951 Truck Shop Manual for your reference.
If you're sure the adjusters are turned in, you might have to manually push the shoes inward to slightly compress the wheel cylinder. Be sure to place the drum on squarely over the brake shoes, otherwise it will bind on the brake shoes. It's a tight fit, but when everything is lined-up correctly it should slide over the brake shoes.
Let us know how it goes......
