The first parts that fail most often for clydes is the wheels. The cranks aren't loaded that much more with heavy-riders than light because of pedaling dynamics. The extra weight may be used to push down on one side, but that force is also being used to push the dead-leg and body up on the other side. So it balances out and the resultant net-force on the cranks is due primarily to muscle-exertion.
The frames can typically take 1200lbs+ before they bend noticably and 1500lbs+ before they take a permanent bend.
so wheels are the most vulnerable. You'll want heavy-duty 36-hole wheels built on strong rims. Then you're talking about 400-550lbs of load-carrying capacity. Tyre-pressure and size plays big role here to prevent pinched-flats. Used large 32mm tyres at 100psi+ to handle the load.