Carthorses are not made for carrying so much weight. There were breeded for pulling trees etc. and the constuction of their ridge is not half that strong than one might think.
They are "weaker weight carriers" than most people think.
As kristy said, heavy overweight isn't healthy for any horse at all (think of kissing spines and so on).
Warmbloods might be the better choice. If you coose a really large warmblood or maybe a warmblood-Carthorse-Mix, the chances are better. Warmbloods are being breeded for the use of riding, therefore their ridge-constuction ist a bit stronger- even today.
A "heavy warmblood" seems to be better than a "light carthorse"...
See here for example: (hope, it is allowed to use this picture from:) http://www.g-e-h.de/geh-raku/pferde/pferolde.htm