Hi Michael,
Well, to be precise, as though you were editing an electrical engineering manuscript, the proper term is standing rib roast. Prime rib would have to consist of "prime" grade ribs and you are not likely to find them nor be willing to pay the price.
In the FAQs is the method for standing rib roast and it is simple and rather fool proof, if you don't over cook. Trim the rib section of excess fat. The chine bone should be removed by the butcher. Bring to room temperature and rub well with a mixture of 1 T. salt, 2 t. each garlic and onion powder, 1 t. each ground bay, ground thyme, fresh ground black pepper. Place on a rack over a shallow pan containing about 1/2" of water to keep the first juices from burning. Place in a preheated grill at 350° and plan on 12-15 minutes per lb. BUT check the internal temperature with an accurate thermometer at least 15 minutes early. Remove when the internal temp reaches about 130° and allow to sit for 12-15 minutes while you de glaze the pan and adjust the seasoning for Aw Juice (Southern Speak) gravy.
Slice and serve on warm plates with gravy on the side.
I would really recommend that you look at some more plans before deciding on the "Big Boy." "The Great American Barbecue & Grilling Manual" has chapters on design considerations, with plans and illustrations of several grills made of 55 gal. drums. Check it out in the Barbecue Store.
Try the yellow pages for "Cooperage" for new drums.
Luck,
Smoky