When I give it half throttle or more off the line, there is a loud clunking sound under the front passenger footwell. The transmission and all related parts check out okay. Any ideas? Thanks

 

If you checked the tranny mount and it's fine you snapped a motor mount.

 

If the motor mount is snapped wouldn't the engine move around if I put it in gear and tap the gas while holding the brakes? It doesn't.

 

Sometimes yes sometimes no. When your holding the brake the motor won't flop as easy. Jack up the tranny and see if there is any seperation on one of the motor mounts.

 

I jacked up the transmission and the whole car starts to raise, so I think the motor mounts are good. The sound is a clunk, and then if I keep it floored, it buzzes. Could this be from the exhaust system being bent and hitting the floor pan?

 

The whole car will raise. Did you disconnect the tranny mount? All you have to do is jack it up a little and see if the space on one mount is bigger than the other. It could be your y pipe or something but if it never did it before that would take you back to motor mount. I can't believe noone else has any suggestions.