I'm not a Clydesdale, but I had to get a larger saddle to relieve hip arthritis and my first impression is that it's the best seat I've ever ridden on. It's the Selle SMP Martin Touring, now on sale at Nashbar for $40: http://www.nashbar.com/bikes/Product...90_-1___202449
Width is 218 mm. See my review in 50+ forum. Highly recommend for larger people.
There aren't many reviews of this seat online, but a couple of them mention successful high mileage.