Torque Rating

[Spirator]

You'd be able to lead them up to the material's yield strength. Most of the force on a worm gear would come from force along it's axis coming from the backdriving of the output gear which would be one mode of failure. As previously mentioned with a heavily loaded gearset, you'd also encounter a lot of friction and this would cause your motors to work harder so make sure you lubricate those gears!

The wormbox is made of nylon and while nylon will yield at less stress than steel, it wears less and a nylon gearset will also have less friction. In this application where you're unsure whether or not your gearset will hold up under loading, you may consider it safe to go with the McMaster parts.

As for a more direct answer, it's difficult to calculate how much torque they can withstand without detailed calculations. The 57545K527 worm is made of steel and it should be able to handle the loads fine. If you have doubts, I'd recommend to fail early and fail fast, or just email McMaster Carr asking.

Edit: I see you already emailed them! Fantastic. 340 in*lb or ~38.41 N*m isn't that much. How much load are you placing on the output of the gearbox?