I guess you'll have to put back your stock motor mounts!
Or call Brenspeed, and see what they have to say about it.

Is the car running now? How do you like the Paxton?

 

love the paxton!!! but i cant stand the rattling noise im getting. guess ill be putting the stock motor mounts back in if i dont get a better solution. but i love the prothanes!!! really dont want to pull them back out!!

 

I had a problem like this not on a mustang but I used exhaust wrap and 2 clamps to cover the pipe in the small area that they touch. Did it temp but wound up leaving it