I'm with the Turtle on this one. A full rack of spares has lots of delicious meat that really isn't "rib" but that is still very tasty when properly used.
To trim into St. Louis ribs, cut off the "flap" that is obvious then with your finger, find the line of joints that run through the rack lengthwise parallel to the factory cut side. A boning knife will easily allow you to remove the excess "non-rib" meat and you'll have a nice, squared up St. Louis rack.
Rub and smoke the flap and extra meat at the same time you're doing the ribs but monitor the temperature carefully because it will often cook faster than the boned rack and dry out. Shoot for an internal temp on it of around 180. Since it usually has less internal fat than the boned portion it may be a little chewier but still has lots of porky flavor. Use it in beans, slice it for sandwiches, gnaw on it for a snack, whatever you'd like.
Individual Sam's Clubs vary but sometimes you can get the meat department to do this job for you.
Hub