Are these American breeds? Not heard of large breeds, but American/foreign breeds shows often have pleasure horse classes either in English or western/traditional tack and costume.
The good news is that you don't have to do an individual show! All work is done on the rail (track) as a group except that you may be asked to rein back from the line up.
There is a possiblity you'll be asked to do a individual show I'd reckon. I'd reckon pleasure horse is meant for things that may be a bit less conformationally perfect but are enjoyable, well mannered rides. So you might be asked to show three controlled paces and a small jump? Large breeds possibly going to include clydesdale crosses, pures etc?
That's how I'd read it anyway. Why not contact the show secretary and ask them what is expected?