Since the Shire is a rescue, the quality of his coat is most likely compromised from feed quality and amount. Feeding him adequate amounts of hay will help keep him warm and you will have less winter coat. Feeding him fats will also improve the quality of his skin and hair as well. Rarely do you need to body clip a draft. They have a higher % of body fat and just don't grow much of a coat. If you're showing to non draft judges, I'd meet them half way. Clip his face like you would normally a light horse. Ears, muzzle, bridlepath... not always done with Shires and Clydes. You will want to blend the stray hairs down the back of the legs to the knee. They will frown on the feathers but that's there problem. They must be impecably white. If they are severly stained, you might want to shave his legs and start all over again as long as you have time for them to grow out (at least 6 months). You will get really good with blending.
I've body clipped with my everyday 10 year old A-5. Get the wide 10 or 7 blade. It will take you a couple hours and sets of blades to do a draft. Cut against the hair (up) and you will have a consistent length. Do it at least one week before the event. I 've tried with my Clipmasters but they leave lines that the smaller clippers don't